Bikertim,
Before you go spending a bunch of money on completely unnecessary parts...
... do you have the basic vacuum booster now (and is it functional) or just manual brakes?
... when you did the disc swap did you get the right proportioning valve for disc/drum?
... have you bled the brakes lately?
... are they just old and worn and need basic service?
... are the rear drums doing their part, are the self adjusters adjusting?
The reason I ask all these questions is that your system should work very well with what you have (unless you are carrying very heavy loads maybe).
If it isn't a race truck and you aren't carrying heavy loads down steep hills, just get a normal vacuum booster if you don't have one and service the brakes for whatever they need, they should be plenty good.
