Charlie,
Welcome to the group!
I have a 40 gmc. I plan to use the stock axle with disc brakes as well. I will be using a 302 gmc 6 in my truck. Although I hate to send people down this route... a mustang II setup and a beefed up frame might work better in your application.
These trucks were designed to go 45 to 50 mph and had approximately 85 hp. A V-type engine with better than 300 hp is probably more than that old frame can take without twisting.
If you do use the stock axle... do the tighten-up. change the kingpins, tie rods, bushings, and bearings. Buffalo Enterprises can make disc brake brackets for your axle.
good luck
John
