I had a similar experience with my 2-ton a couple of years ago. It turned out the pool pump had failed and took out a couple of rod bearings along with the crank journals. I finally located a '53 235 that would bolt right in. I had to have a custom exhaust pipe made because the manifold commotion is at a different angle, but the pip was shot anyway. My recommendation if you change the engine is to pull the sheet metal off the front of the truck (it comes off in one piece with just a few bolts) and you can get right at what you need to and even do a test start with a garden hose supplying cooling before the body parts go back on.
Any 6 "54 or older will bolt right in, the newer ones need a water pump adapter to work with the stock radiator position. As mentioned above, a V-8 will take a great deal of engineering to work; there's a reason Chevy reengineered their whole trucks when the V-8 came out.
