Floor pan replacement

[Msbaugh440]

Got my interior pulled apart. The floor pan is in really bad shape, worse than I thought at least. Looks like the whole firewall is good, but the floor pan is pitted too bad in some areas to just sand down and re-paint. I'm thinking about fixing it the right way the first time, using one of AMD's full floor plan replacements instead of butt welding patch panels.  I've gotten really good at welding patch panels, but don't think it will be the best fix for this one, since there's small rust spots both in the front and back of the floor. I have never done a floor pan on one of these things and have a few questions...

• Can it be done without pulling the transmission? I don't think I'll be able to get to the spot welds on the front firewall flange.
• Any other tips or suggestions for bracing the car before cutting out the entire floor? Is having it on 4 jack stands on the frame good enough for the job?
• What, besides weld-thru primer, is the best way to seal the flanges where I'm spot/plug welding the new floor in? Just slap seam sealer on the flange? What about the underside of the flange underneath the car?
  

[truckinman466]

I don't know about the tranny question but as far as your bracing goes,as long as you are level you'll be ok. I always use weld through primer between the panels. You should seam seal after you get a primer sealer on first. I just did my car firewall to tailpanel. One section at a time, you will be good

[kathyscuda]

take it off the jack stands. The suspension will keep it level.
grind the welds around the hump. Remove it. take out the 2 bolts that hold the shifter mechanism. let it hang out of the way.
get your air chisel with the flat blade and start cutting.

[xx88man]

The AMD pans fit 71-74. If your car is 1970 you have to leave a flange to weld to. Just thought I would point that out as you didn't mention the year of car you're working on.

[aussiemark]

You can get 1970 specific floor pans that fit perfect.

[Msbaugh440]

Ok, leave it on it's wheels, not on jack stands, drill out spot welds on top of frame and flange, remove transmission hump first, make sure I order a 70 specific floor pan as they're different from 71-74, and use an air chisel to separate from flange. Sounds like a good starting point!

[Msbaugh440]

Here's what I've got it down to so far, for the most part there was only surface rust on the firewall. I do not think the floor pan is salvageable with the pitting I'm seeing already.  Leaning towards full replacement but I'm going to grind/sand it down all the way

[Rich G.]

I use a disc sander and grinder and ground off most of the metal on the spot welds so when I hit it with the air chisel it popped right off and didn't cut into the frame rails or rockers.

[anlauto]

You should strip the car entirely, have the shell media blasted properly, then worry about fixing the rust 

[Msbaugh440]

I know you're 100% right about that... I should do the entire thing properly, but the rest of the car is in great shape. The floor pan and the trunk floor are the only spots with rust and the paint on the car is still in great shape, so I'm not quite sure I'm ready for the full blown media blast/repaint work.  The car is not an original build, so I'm basically just trying to get it back on the road

[jimynick]

It took some finageling to get my AMD full floor in and I had the firewall out at the time. The front flange that meets the firewall flange could be bonded if you can get some clamps in and on it. Failing that, you can weld it together from the inside and seal the outside to keep water out. I wouldn't try to keep that oem floor; by the time you grind away at the rust you'll have it as thin as tissue and you won't be happy down the road. Check the outriggers that weld on the inside of the rockers and carry the sides of the floor. If they'tr iffy, now's the time to buy/fab new ones before you get the floor out. A box of Clecos can also be your friend when doing this and expect to have to "massage" something somewhere- count on it, but it's doable and give 'er a go! 

[Mopar5]

My own humble opinion here if the passenger side of the floor pans are good , I would just section the AMD panel for the areas you need only instead of removing the entire side.

[70 Challenger Lover]

I would section repair it too if it's just one area. Butt welding is harder and you will have to clean up welds with the grinder but it should come out nice if you take your time. In the end, probably amount the same effort as replacing the entire floor.

Another route to consider is patching it but overlapping the metal rather than butt welding. You can get away with this method since it's all covered with carpet but you need to use a good sealer at the joints in between the overlapping metal. This will be significantly faster and easier. Overlapping is basically what the factory did at all the seams so it's not considered substandard work. The rear foot wells are panels that are overlapped with the front floorboard.

The hardest part of replacing a floor in my opinion is to get the front edge to seam up nicely with the firewall panel. Doing a section rather that the entire floorboard may be a lot easier to fit properly along this leading edge.

[jimynick]

Please, don't section/patch it! While the factory did overlap panels, it was only at the edges where they met and the first sign of a hack job is an overlaid patch. As I mentioned earlier, if the panel is rotten enough to replace in the noted spots, how much behind can the rest of it be? You'll go to all the work and expense only to end up with a substandard repair that will be a philosophical stone in your shoe which will detract from your ultimate enjoyment of the car. I got my bodyman's licence in 1974 and have been there, done that; but in the final analysis, it's your car and your call. 

[Msbaugh440]

Woah! Thanks everyone for the advice, but wow, a lot of conflicting opinions!  Don't worry, if I were to section anything it would only be butt welded, no Laps. Turns out I need to put a little more work into this thing though, so I think I'm past the point of sectioning metal in.  Looks like the rear foot well area needs to be replaced on both sides too, the passenger rear foot well already has pin holes after grinding it down some.  All the metal on the sides near the rocker and under the rear seat appear to be in great shape, so I should be able to tie the new floor into good metal. I would only retain the two factory Laps where the stock locations near the front/rear pan overlap and the rear/under rear seat overlap is.
Is there any way to get rid of those lap joints as well?  I'm looking at AMD and looks like I would have to buy the complete front and 2 rear foot well sections separately, correct?

I think entire new metal is the best (and easiest?) way to do it since there's really only plug/spot welding to do. I'm going to have to finish stripping the entire interior and looks like the transmission and drive shaft are going to have to be removed as well for this job. Sounds like I'm ok cutting it out with it on it's wheels, but I'm seeing different opinions on bracing as well. I do have sub frame connectors so that should help with rigidity. 

I bought a nice Blair spot weld cutter and an air chisel so that should hopefully get me started.