Drum 2 Disc swap: Which master cylinder do I need?

[thecatseye66]

Thanks for checking to see if you can answer my question.
I just finished changing the entire front crossmember from a 1977 2500 Trailering Special into my '68 C-20 396 that already had an original power booster. Now, I can't get the brakes working right.
When I took the parts off the '77, I took the booster and master cylinder, all the brake lines and the proportioning valve from the donor truck too, but I couldn't use the booster or the m/c from it because 1) booster mounts differently to the firewall and it has a solid, long rod that connects to the pedal rod directly through the firewall (instead of the booster already on the truck which has a lever-type connection), and 2) booster/master cylinder has a short pushrod setup that won't let the m/c work with my original booster.
I used the lines and the proportioning valve from the '77 but I used the original master cylinder and booster from the '68 and, well, it isn't working right.
I bench-bled the m/c and I've got a solid stream of fluid at all four bleeders (a lot of volume from the front calipers), but the pedal goes all the way to the floor before it will hold the truck at idle.
I'm pretty sure I need a master cylinder especially for the disc/drum configuration (is it because the the discs need more pressure or volume?) but I don't know what one to buy that will work with the stock '68 booster?
Will the master cylinder from a 71/72 disc/drum bolt up to my '68 booster and solve the problem?