BJD....if you dont want to spend the bucks for the DISC brake upgrade use the larger drums off a Duster.....all you really need to do is change out the wheel cyls and the shoes & hdw...... the larger DRUM brakes are 1/4 the cost and they WILL stop your car......the only thing I strongly suggest is that you replace that "suicide single" Master Cyl with a dual also from a later model duster and bolts directly up..... Unless you are drag racing the living **** out of your car and beating it to death from every light, the cost for large drums brakes that bolt directly up to your spindle vs the thousands for disc set up is not only not necessary it is not cost effective.....I have a 66 340 with large drums on all 4 and dyno'd at 358 HP my car stops NO problem....BUT I dont do the following: I do not run my car at the strip, and I do not beat the living crap out of it from every light.....I still get into it JUST NOT all the time ....I did not spend that much of my life restoring my 66 to the level that it is at, only to abuse it.....just not my thing. Are disc brakes better you betcha are they also 4 times the cost? you betcha!! will appropriate drum braks stop your car...YOU BETCHA!! Drum brakes have been stopping 5000 lb LAND BARGES for almost 100 years and they work just fine for "regular use" and the occasional burnout of a 2700 lb A body....
unless you plan on rallye racing or drag racing or racing of pretty much any sanctioned kind...you do not NEED disc brakes....YOU do however NEED a dual Master....and they are as easy to retro into your existing system as it can get....you DO not NEED a proportioning valve, just a splitter block ( which is likely already there ) good to go and at 1/4 the cost
my 2 centavos
