Front spindle differences

[gzig5]

I am looking into aftermarket brake kits from Wilwood and I see that they have options of 14", 12.8", 12.2", 11.0" or 10.75" diameter rotor when used on a 70 drum spindle. Two piece rotors are available for some of the larger sizes as are six or four piston calipers.  Only option for a 70 disc spindle is an 11" four piston kit. My car is a 73 and has disc spindles and for this setup they have four or six piston calipers, but limit the rotors to 12.2, 11.0, or stock 10.75".

So, on to the questions.  Obviously the disc and drum spindles offer different mounting options and the configuration of the 70 drum spindle must allow the most flexibility to fit a big rotor and caliper.  Firstly, can the 70 drum spindle be mounted in place of my 73 disc spindles if I wanted to do that?  What are the other differences in the spindles that I may need to be concerned with?  Bearing size, ball joint size, other?? 

I will be running auto-x and a couple big road course tracks in the area including Road America.  I want to employ as much braking capacity as I can and generally with brakes, size matters.  Now Wilwood may not be the best option but I'm starting there because I've come across a six piston 12.8" two piece rotor kit from a 2000 Mustang for what seems like a great price.  Would need new brackets and probably rotor hats but it may still be quite a bit cheaper than buying new.  I know that Dr. Diff has some options and there are probably a couple others I've missed so feel free to chime in. Any "danger" with racing on a drum spindle from a loading perspective?

[HP2]

Inner bearings are a different size between drum and disc spindles, but that is primarily in the ID. Disc's ID is larger.  The '70 drum spindle was used on all B bodies from, I believe 62-72ish, so it has a wide range of availability. Yes, you could use a drum spindle in place of your '73 disc spindle of you wanted to so long as you use the correct inner bearing. Outer bearing and ball joints are the same.

For autocross, big braking is less of a need due to the speeds. Smaller and light weight with only enough mass to do the job is better there. Times are short and repeated hard breaking has plenty of cool off time between runs, if competing.  Road courses are the exact opposite, but even with larger rotors, cooling and pad compound can be bigger factors for open track days. Wheel to wheel racing is a whole 'nuther deal, but I doubt you are doing that with your E body.

Drum spindles are kinda puny looking comparatively, but they are forged steel and pretty robust. Even in my oval track days, I never saw a spindle break. Ball joints, yes, seizing wheel bearings, of course, but outright broken spindles, none that I remember.

I seem to recall that Wilwood used to offer a 12.125x1 rotor that would fit in 15" wheels for oval track applications. Those guys see some huge lateral loads and high heat range. Of course a good gripping 15" UHP tire doesn't exist unless you use racing tires. Regardless of how large a rotor you go with, your braking power grip will come down to tire choice. No point in using a 14" rotor if you are using all season tires.

[Mr Cuda]

I would ask how much weight, and what kind of road speed do you expect to be braking from?
It comes down to swept area and brake mass for repeatablity.
I have a standard 10.5" 1970 spindle swap 4piston wilwood set up.
I think a standard factory disc stops better. Don't know about the bigger wilwood setups.
I upgraded to 73'B body spindles (taller than 73-4E, big inner bearing) and C body rotors. C body rotors are 11.75 x1.225 but if they were aftermarket they would include the part you can't use and call them 12"
I've only had to brake from 120 to zero a couple times. They stopped.
My few trips on the road course were with standard 10.5", and they performed well. Didn't really brake much except the downhill, 100mph,to 30 for the esses.
I have german cars with 13"and14" brakes. But they are big and heavy, not like a average 3400lb mopar.

Here is your standard 10.5 wilwood, and the pad difference.
Aftermarket look cool , but removed for factory.

[gzig5]

I haven't driven on Road America yet so I'm not quite sure of the top speed.  But I used to have a 2000 BMW M5 that I was planning on taking there and in talking to a friend who ran there regularly, he thought the car would be doing close to 140mph at the end of the big straights.  My Cuda is lighter (3700lbs?)  and more powerful than the 400hp the M5 had so I think that 125-140mph is entirely feasible. 
I'm fairly aware of the physics involved with performance braking systems from owning the M5. It's top speed was 178mph (I never got that high) and she's a big girl at around 3900 pounds.  So the brakes they use are a good baseline IMO, and they stand up to Ring Taxi duty in Germany with upgraded pads.  I'll be running the stickiest tire I can get and will be at least 275 wide on the front.  I'm considering having a second set of wheels with dedicated track tires.  I'll be adding cooling ducts and want to run the largest, thickest rotor I can for the thermal mass.  If I remember correctly, the stock M5 rotor is 12.8" x 1.3" or thereabouts, so I'm shooting for something in that size range. I actually still have a set of rotors and calipers from that car that I'm going to mock up this spring to see if I can adapt them. 

I may never test the limits of a proper BBK but I would rather over-build the car and never experience brake fade, than under-build and have the frustration.  I'm at the point in my life where I need to do it right the first time because there may not be the time or opportunity to do it over.  It's usually cheaper to do it right once than replacing an underperforming system.

You mention using 73 B-body spindles that are taller.  How does that affect the geometry? Does it act like a dropped spindle, lowering the front?