Did something similar, LS1 BU ..
I did the Global West front end with LS1 12" brakes up front, and Ford 9" in rear with disc brakes (that I believe came from an Explorer). The car handles much better than before, and except for slight wheel rub on full turns and large BS of front rims, the conversion would be excellent. Rear brakes don't hold lot of boost, though, and overall the braking has not really impressed me for what is there. I was told by GW that the stock PM would handle it, but not so sure now that I read this thread with your results. Pedal travel was not an issue. I have RedRegalT's vacuum brake kit to go on, but not sure of all that's in it as far as MC and so on. Any thoughts as to how to go once I get in there so I do it right first time?
PS Because of other issues with 9" rear end, specifically with the Detroit Locker, I will be putting stock 10 bolt with drums back in for a while.
I looked at RedRegalT's kit I was sent, MC is GM part #18010023, which corresponds to an '86 S-10 pickup MC. Looking at your NAPA MC online, it looks exactly like the one I was sent. So, I should be good to go once I get it in there. The kit has booster, MC, pedal, and pushrod, instructions, and it is all clean. Looks to be just what the doctor ordered.