Drum brakes can be awesome especially at normal speeds. They are self-energizing so as you come to a stop, you usually have to begin decreasing the pedal pressure. I don't know why yours would not be adequate.
I'm pretty sure I found replacement parts for mine.
Best bleeding IMO is the old fashioned way. The disks should be gravity bled if your patient (and lucky).
For the rear lock up, go over the drum system to make sure everything is up to snuff. Nothing should be grabby. Make sure the front linings have the same or higher friction ratings (marked on the edges or back as a two letter code). Smaller rear wheel cylinders may be in order to reduce the force. Then when you go to front disks, you can reavaluate that decision if need be.
Look at the newest Brake tips here: Master Technician Service Conference - Chrysler's Training for Mechanics
Drum hardware was changed '68 to '69 so those details from anybooklet '68 and older prob won't apply to what's on your 8 1/4
mymopar may have the same and some of the newer MTSC booklets the Imperial Club doesn't have.
