If I had access to all the parts etc you do I would use the 66 frame
because it is level in the area under the cab while the later model frame has a dip there
I would keep the axels from the later frame and put them under it to have disk brakes
along with the power steering
If money is a factor you can even use the wiring from the later model truck
steering box :
there is a mount for sale on the 66 board I believe to make it mount up
If you go with a 454 you may have to set thhe firewall back or move the motor mounts for that reason I perfer a small block
Gene Sharp