Background:
I swapped 11" discs onto the front of my '64 Dart and left the 9" drums on the rear. I also have what I believe is a parts-store '76 A-body master cylinder (dual reservoir), which I had installed for the sake of redundancy, long ago when the car still had 4-wheel drums.

Like an idiot, I slapped the disc brakes on this year without a proportioning valve and took the car road racing. It actually worked pretty well, but I suspect that the only reason we never locked up the rears is because the car is so slow we never really had to get on the brakes hard enough. To be safer for the next time around, I'm now going to re-run the rear line and install a driver-adjustable proportioning valve on the tunnel somewhere.

Question 1:
With brake proportioning in place, is there any reason at all to upgrade the rear brakes? I don't need more stopping power in the rear, but I figured that larger drums and shoes would at least last longer when properly adjusted. That is: more contact area, dialed down with the prop valve, could apply the same braking force with less pressure. So I wouldn't have to go through the hassle of swapping shoes as often, and the drums would probably last longer as well. Is this worth doing?

Question 2:
If the answer is yes, how difficult would it be to use the 9" drums and shoes originally made for the front (2.5" wide) in place of the rear (2" wide)? I gather from reading previous topics that I would want to retain the smaller wheel cylinders used in the back.

Notes:
Yes, I do still have the small bolt pattern on the back. I'm using billet aluminum adapters to run the same wheels all around. The long-term solution is to find a later 8.25" or 8.75" rear that would have bigger drums anyway. The idea in this post is just a cheap, short-term idea to keep my rear brakes alive and lower-maintenance. Our next race is in 4 weeks, and the track is supposedly very hard on brakes. I'm already stocking up on spares for the front discs.

Feel free to lambaste me for overthinking this, or for a variety of other reasons.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

with a good disc/10" drum setup no prop valve is necessary.

In your case I would leave what you have alone....and focus on a BBP 10" drum 8.25 rear axle as soon as you can.

Waste no energy on the little axle and brakes.

On my 65 Valiant I installed a Ford Explorer 8.8 with discs .......you would like that a lot.

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Yeah....Im the one who destroyed this rare, vintage automobile.....