Thanks for the encouragement! I can't wait to see your red truck come together hardwaa...
Waiting on schedules to align with my buddy so I got a few more projects started. I installed the little brake booster. I used a Speedway 7" booster, and I ordered the 55-57 brackets along with the aluminum plate adapter that I used on my C10. Turns out I didn't need that, since the bolt pattern on the GMC matches the master cylinder and the 55-57 brackets (I should have realized this since I had already replaced the MC). I could still add a couple of bolts but I don't think I need it, the pedal force is all in line so it shouldn't flex. I'm waiting on a pedal clevis and I'll see how much it moves around once the pushrod is connected.
The truck was converted to an SM420 a long time ago, and it already had the removable floor panel, but it never had the firewall filler panel swapped over. I can confirm that the panel that's advertised for the 60-63 trucks will fit in the 64-66 trucks. The only difference I found was that the area where the accelerator pedal is mounted is flat for the pedal that bolts through the floor, rather than the style that snaps onto the ball studs. Maybe this is how all the 4-speed trucks would have been for all years?
I ordered a new pedal and a rivnut tool - I'll use rivnuts to hold down the floor panels too. I'm not going to weld in the firewall filler panel - it's way too convenient to have access to the bellhousing bolts.