Rear dics brakes??

[454_72_4X4]

I would like to put rear disc brakes on my 72.
Thought maybe some of you guys could provide me with some answers.

First my truck specs;
72 3/4 ton 4X4
Mid 70's Corp 14 bolt rearend

Has anybody already done this swap?
I've looked at some of the "kits" and I just think they are WAY overpriced.

I've already got a set of front discs and backing plates.
The kits I've seen have a spacer between the axle flange and the disc backing plate.
Any ideas on the dimensions ot the spacer?

Parking brake will be a hydo-lock so that's no concern.

Also I have an aluminum mastercylinder off of a mid 70's truck on my 72, (with a late 70's Monte Carlo power booster!).
I know there's a residual, (not sure about the spelling!), presssure valve in all brake systems and that a drum brake system requires one with more pressure than the disc brake system does.
My question; whereis it located and how would a person go about changing it?
Any other things to look into about this swap?
Thanks!