[1project2many]

If you get "everything" then it's probably a good deal. Plan to spend a bit if you're going to refresh the engine. I'd look at GMC school buses through mid '60s as some used Pontiac engines (Some used Olds engines, also). Do you have manual transmission? Old buses were usually spec'ed with manual transmissions although a large truck trans will usually have a different bellhousing than a light duty pickup.

How about a GMC V6 from a later truck? That's an "all GMC" engine and there are plenty of folks still building them so you might be able to find parts. There must be an old grain truck around with a 351 or 379 or even a 60's light duty truck with a 305?

Factory built V8 truck generally differ from 6 cylinder trucks in throttle linkage, fuel lines, exhaust, front and rear engine mounts, possible front crossmember modification or bracket change, radiator size, fittings, and location, bell housing change, wiring changes, starter rod elimination or change. If you're going to go with V8 I'd say spend a bunch of time locating original pictures and service manual pages to ID differences before jumping in.