The only "correct" engine for your truck would be a 1955 year 265, since that was the only year the Chevy V8 was produced without an integral oil filter. The 55 filter was a bypass unit similar to the one used on stovebolt sixes. 56 and up engines, 265 and 283, had a canister-type filter at the driver's side of the bellhousing area. The chances of finding a 60+ year old engine that doesn't need a total rehab or hasn't been rebuilt multiple times already will be somewhere between slim and none. Then there's cylinder heads- - - -"period-correct" heads will need to be made before 1960 to have the correct spacing for the valve cover bolts. Once again- - - -rebuildable heads are scarce.
Swapping to the 350 gives you a much bigger pool of suitable engines, and an automatic trans with overdrive and/or a locking torque converter will make the truck a lot more highway-friendly and give better gas mileage as well.
Jerry
