Trunk Floor Replacement

[Roberts440]

My '70 Challenger had a two-piece trunk-floor replacement performed 20 years ago.  Now, this replacement is rusted, the extensions are rusted, and the original floor between the rear frame crossmember and tailight panel is rusted.  The lower rear quarters are also rusted.

I've seen the excellent help and comments on here concerning this, but I would like to revisit this.

I have ordered a one-piece floor, extensions, and quarter patch panels.

It looks like there are two main methods of doing this:
1) Remove the tailight panel, and slide in floor over the crossmember, or
2) Remove the crossmember, and slide in the floor under the tailight panel

A third method requires removing most of either quarter and the inner/outer wheelhouse, but I'm planning only on patching the quarters.

Other posts on here appear to show sliding the floor between the tailight panel and crossmember, but the gap between those doesn't look wide enough to pass the floor's shock humps through.

I'm hoping not to replace the tailight panel, although it will require some patching along the edge where it meets the floor panel flange.  If I use the entire quarter patches, I will indeed need to remove the spot welds between the quarter and tailight panel, so perhaps totally removing the panel is not that much more work?

Removing/replacing the rear crossmember does not look like a trivial chore either.

So, any suggestions?

Another matter - I've done some plug welding before on a passenger floor replacement on another vehicle, and I recall wishing I had a spot welder.  Any comments on the cheap HF type of 110v spot welder?

Thanks very much in advance!

[Dmod1974]

The HF spot welder is just about useless; I have one, tried it once early one in my resto, and quickly realized it was not possible to get consistently good weld penetration with it with thin sheet metal, let alone anything thicker like the rear crossmember.  Get good at plug welding and you'll far exceed the weld strength of anything that welder or the assembly line ever was capable of back in the day.

I vote for removing the crossmember since you are already doing the trunk extensions and full floor.  The crossmember comes out very easy and is very easy to reinstall since you have the bumper bracket bolt holes to help line it back up - tail panel is a lot more work IMO.

[jimynick]

Ever the contrarian, I'd pull the rear body panel. There's at least two reasons to do it what with the floor and having to repair the panel anyways. It'll also allow you to get in the trunk area to properly weld where it needs it and installing the trunk extensions will be easier, too. Don't forget to R&R the gas tank supports while the floor is out. 

[71chally]

I would also take the crossmember out if I liked the trunk/rear panel fitment.  Climbing over the tail panel ain't so bad.

I would agree on the trunk extensions though.

[chargerdon]

Curious...what made you decide on a 1 piece replacement floor?   The two piece replacement works SOOOOOOO much easier, the the seam isnt bad and disappears once you put down a new trunk mat !! 

[Roberts440]

Thanks for the replies.  The HF spot-welder report is a downer.

I decided on the one-piece replacement after reading a Motortrend story (https://www.motortrend.com/how-to/how-to-replace-a-1970-dodge-challenger-trunk-floor/).  They had removed the entire quarters, tailight panel, everything, so of course it was easier for them.  I just received the last of the AMD order last night, so I'll continue removing the old floor.

In the next few days, I will make the rough cuts into the original quarters where the patches will go, and also remove the spot welds between the lower quarters and the tailight panel.  Then I'll decide whether to remove the tailight penel completely, or instead remove the rear crossmember.

[69 500]

you can put a 1 piece trunk floor in the opening , think outside of the  box,
    cut the back edge off of the 1 piece floor ( where it extends to the out side of the tail panel) leave approximatly 2 inchs of old floor inside of the trunk in front of the tail panel, make a few pie cuts on the edge of trunk floor where it curves around the back side of the wheel house ,( pie cut the edge that spot welds to the back side of the wheel house) this allows you to fold the "wings" that extend around the back of the wheel house and spot weld to the trunk extension up to allow you put the trunk floor threw the trunk opening. dont forget to remove the trunk lock support ,once inthe trunk area place a suitable support ( 2x4) under the folded area of the new trunk floor and push the folded area of the trunk floor back to its original position . becareful not to crease the "wings" when folding up . put into place maybe a little dollie work on the folded areas depending on you crease . clamp into place AND WELD . hopefully you were able to drill out the the spot welds on the trunk extension from the framerail side of the trunk extension,the you can plug weld to the trunk floor.

[Brads70]

While at Carlisle I was taking to the AMD rep about doing the floor and he suggested with the one piece floor to cut it over the one frame rail so it fits in the opening,  then weld it back together over the frame rail so it can't been seen . This avoids having to remove the tailpanel. Timely post for me as I have all the parts to do this as mine needs to be done but have no experience doing it....

[DeathProofCuda]

Maybe I'm missing something, but even if the HF spot welder worked well, I can't see how it would be much help in installing a new truck pan, except for maybe a few spot welds along the outside edges of the quarters, or similar spots.  There's no way that the jaws on that thing would have the reach to access all the centrally located spot weld locations of a trunk pan.

[Roberts440]

Thanks again guys!

I wanted the full floor pan because the section of original pan between the crossmember and the tailight panel is almost totally rusted out.  So, I can't use the two-piece pan, nor can I cut that bit off of the rear of the one-piece pan.

I'll have to look closely at the suggestion of cutting the pan in two at a frame line.  I assume the idea would be that you could then slide the sections in from the trunk side, sliding the rear lip under the tailight panel.  Brads70 - if you try this, please post!

Regarding the spot-welder - I thought I could use that on the pan-to-tailight panel weld, pan-to-extension panels, pan-to-wheelhouse, extensions to wheelhouse and quarters.  But if it doesn't work...

[tparker]

My trunk was rotted through as well. I put in a two piece trunk bottom and it sucks. Or my installation of it sucked. I have thought about replacing it but don't want to rip the car apart as it is a lot of major surgery. One option I have thought about is keeping the edges which are mostly in good shape then cut out a one piece floor pan to fit that section and weld it into the existing piece. Not sure if that would help you at all, just depends on how much of your existing floor is salvageable.

FYI, I have a Harbor Freight spot welder and I could never get it to work right. the arms don't line up right and it there is a throw switch that has to be switched on then off which doesn't give precise control. I bought a nice cheap Hobart welder and it did a nice job for simple stuff. About $200-$300 or so. You might be able to rent a welder.