So the bad news is that you can't just mix and match brake parts and expect them to work. The front pistons size needs to be matched to the rear piston size in order for the brakes to be properly balanced. The correct ratio of front size to rear size is a bit complicated since it depends on vehicle weight distribution, how tall the center of gravity is, tire size, etc. but the usual rule of thumb is that you want the front brakes to do about 2/3 of the work.

I don't know much about the Eldorado brakes but my guess is that the pistons are too large and that you'll never get that setup to work properly.

I'd throw the GM stuff away and put a DoctorDiff rear disc kit on there. At least the DD rear kit is properly designed to work on a Mopar.