If you go to a hydro boost set up would it be ok to just change the front drums to disk and leave the rears drum?
All you're doing with Hydroboost is adding a more assist to the brakes, so you can even do all drums, if you want. You will need to get an appropriate Master Cylinder and Proportioning Valve for Disc/Drum.
My 77 came stock with a front discs and rear drums, so I didn't even need to change the Master Cylinder and left the stock proportioning valve. This made for a real easy afternoon swap. I felt that some Ford engineer probably knew what he was doing with designing an appropriate MC for the stock braking system. The smaller the bore size on the MC the less pedal pressure, or more line pressure you get, but it's also relative to the calipers, rear brake cylinders and line size. With the extra help of a Hydro Boost, I probably could use a MC with a little bigger bore, than stock for a 76-77, but with 35" tires, I like the pedal feel I have now. It took about a week to get used to the extra stopping power.
With the rear drums in good shape and adjusted properly, I personally don't see the need for rear discs. My low range with my NP205 transfer case is only 1.96:1 and I have an auto tranny, so I need good brakes on steep trails. Anyone that was on Chinaman's Gulch trail at the 2009 RMR can attest that my brakes work great. I can even back down steep trails or hold on obstacles without any drama.
