Yeah, I guess I worded that incorrectly. Wasn't suggesting actually swapping a split case with underdrive behind the 420, thats a total waste of money. H41/42 and an orion would be the answer there IMO. I was just saying that you may be OK with the split case underdrive gearing given the...
That underdrive on the 4:1 splitcase may not bother you with the 2-5 gearing split on the H55f? Would certainly suck with the 420's 3 useful forward gears though.
It would be a sweet drivetrain, but unless you fall into the parts for nearly free, you're going to be spending a serious chunk of change. Have you driven an H42 equipped FJ40? Do you think your F-engine has the power to push you truck in 5th gear? I know the F engine in my '69 with 4.10...