Mounting the master cylinder to the firewall does present some challenges. Mainly, the firewall would require extra bracing to be installed to prevent it from flexing. Using the under floor stock location would eliminate that, but I understand that the boosters will interfere with the shifter linkage.
Something you might consider, before you make changes to the truck, is get it in operable condition. Try driving it stock first, see if you like it that way. Sometimes the mindset of "you have to change it to be safe/make it drivable on todays roads" comes up, and people that are new to these truck get bombarded with sometimes false information. I'm not saying all the advise is bad, only that some of it goes a little too far. I will agree with going with a dual master cylinder, even though I still have the original style non-power single in mine. It's a bit of extra safety. Disk brakes? Nice to have, bot not 100% necessary. The original engine and trans? Bulletproof in stock form, plus teaching your son to drive a manual trans, especially a 3 speed column shift, will give him a one up on his automatic only friends. Thinking about it, even keeping the manual steering and brakes (drums only, go power if you go with disks) might even make him a better drive by making him have to pay attention to what's going on.
Whichever way you decide to, though, feel free to ask questions.
