My daily driver is a '55 1st series GMC Suburban, which means its very similar underneath to your truck.
I have a GMC 270 with the granny gear 4-speed suggested by others. I've got the rear end from a '73 Chevelle and used a kit to put the disk brakes from the same car on the front.
I think the combination works real well, but I also have a 454 I plan to put in someday...
