Stick with a 5 litre, 253 pistons are getting had to find. You'll find that the 5 litre, well tuned and reasonable standard gets good economy unless you pedal hard.
Stick with the Rochester carby and don't be suckered into the holley. They actually work much better than holleys as long as the throttle shafts are not worn. With a couple of simple mods to the carby you can get them work efficiently.
Also, get the 3.08 diff and the trimatic for cheap tranmission builds.
Good luck with it. That would be one cool ride!
I use to work on all types of Bedfords as a mechanic in the late 70s. The HQ conversion would be get.......
Peter B