I think it would be easier to upgrade the '66 chassis. I've got a '66 fleetside with a 350 crate motor, 700R4 overdrive tranny, front disc brakes, power steering and vintage air. All the comforts of a modern truck. You can see it here:

http://community.webshots.com/user/ndnchf
Steve


See my 1940 Canadian GMC:
http://community.webshots.com/user/ndnchf