Hi Dave.
The engine in your truck currently does look like a Chevy 216.
The GMC 228, 248, 270 engines will be a easy bolt in option for you. The only hard part will be moving the front frame engine crossmember forward. You will have to drill through the rivits and slide it forward. Afterwards you can go back with grade 5 or grade 8 bolts to secure the crossmember.
A GMC 302 is not as easy to "drop" in as a GMC 228, 248, or 270. If you wind up with a military version GMC 302 you will be in for a hard road to convert it, but it can be done and many have done so.
What are your intentions for the truck? Are you looking for a mean vintage hot rod? If so go with the chevy 261 or the 302. But don't forgot all of that extra horsepower will do you no good with stock gears in the rear end.
If your looking to "cruise" the truck and stay mostly stock go with a nice used Chevy 235 or a GMC 228. If you must then have one rebuilt. Even with the Chevy 235 or the GMC 228 it will be plenty of power and torque for a stock built truck.
There is a saying from an old truck friend Rob English that I think everyone should follow when starting a restoration of any kind. Work in this order; Steering, Stopping, and going. Without good safe steering, and nice safe brakes the best engine in the world is useless when one of the other components fail you on the first test drive.
I say get the brakes and steering in good order, and fire up that ol' 216 and drive the truck. Heck you might even be happy with the chevy 216 and you could use your money on other repairs like body work ect.
