I have nearly the same truck (Gringo: the '45 2 Ton), I would not hesitate to put a SBC in it this winter. Stuff a 427BBC?, wow, that will really fill'er up. Good luck.
Your 2 spd DP rear should have a 6.03:1 and 7.99:1 ratios if I am correct. Do the math with your final trans ratio and rolling radius of the tire you have mounted and you can calculate the RPM at speed. The cam and torque range of the 427 engine will determine if you are happy with the final ratio or unhappy with the engine booming at speed. I think it is up to you, what will you do with the truck? Like the man said: "How fast do you want to go? How quickly do you want to get to that speed?" It is unlikely you will light-up the 20" tires with even a 427BBC Vette engine, if you rev-it and drop the clutch. But you will have a heck of a hauler. Where is that next gas station anyway?
Best of luck
-Rick M.
