Rear brake uncertainty

Just bought a 68 Barracuda convertible. Originally delivered as a 318 auto. The car has been partially upgraded to a 340 by someone who (it is claimed) is very familiar with this type conversion. The car was being sold as part of a divorce settlement and I don''t have access to the original rebuilder.

Problem: Leak in rear brakes -need a rebuild.

I immediately joined FABO to get access to all this knowledge. My shop is populated with mechanics that were all born just before, or after the car was originally manufactured. They are competent and they like the car - but to them it is kind of exotic, and they have little background to chase things down correctly the first time.

So the car now has 340LA block. The carburetion is now 6-barrell from a 1970 340LAT - the 1970 'cuda Challenger Trans-Am sedan version. The trans has been upgraded to a 727, now disk brakes on the front and a 8 3/4 rear end housing. Wheel bolt pattern is 5 on 4 1/2 - I'm told the axles and bearings are new. I'm upgrading to a new M/C and power brake booster. (I'm 77 years old - running out of manual brake leg strength. This car has been on my bucket list since 1971 when I owned a '69 fastback - but, you know, family and kids.......)

The brake shoes on the rear are 1 3/4. The drum (not the original) is 3 inches deep - that would imply 2 1/2 shoes. The drum fits the backing plate.

Two questions: Could I upgrade the shoes to 2 1/2 using the same hardware and backing plate? Is it possible that the backing plate is actually one for 2 1/2 brakes and the 1 3/4 shoes happen to be just centered on the drum? (I'm confident about the hardware, but not the backing plate).

Also, did the disk brake cars come with a brake distribution/balance valve, and if so - where was it located?

Don't mind my being wordy - I've found the most difficult part of problem resolution is getting enough information to be able to make the correct decision.

Thanks in advance...

Tom