My .02 worth. Back when I had my '68, I was swapping '72 discs onto it. I pull a booster off another truck at the junk yard, bought a new '72 MC and everything bolted up. Never really had a good pedal and it took too much effort to put. After screwing around with this for a while, I ended up buying a booster/MC from Master Power Brakes for this app. It solved all the problems.
It might be more expensive to just buy a whole unit from ECE or Master Power but odds are it will work correctly the first time. Hodge Podging things together doesn't always work out so well.
YMMV. Good luck.
