You're looking at limited options...the only way to truly diagnose that motor is to pull the timing cover and check that, because you need to check the timing chain anyway, and to do a compression test to see if the head gasket is bad. The compression test probably needs to be done first. This is assuming you have coolant in your oil. If its just overheating, it could be as simple as a thermostat. Bottom line, is that no one is going to be able to do a complete motor change for 1500, with a new or rebuilt motor. Further, you would be far better off fixing this one, because you would have had it gone through and would know its good. If you buy one off of someone,yo u don't know what you're getting. So, you can take it to a shop, have them fix it (which will probably be about 1500, without a motor swap, but it will be fixed), roll the dice on a mystery motor and have someone put it in (which will also be about 1500 most likely and it may have other hidden problems), or just cut your losses and sell the truck as is and get something else. I'm not trying to sound rude, but these trucks (or anything this old actually) are not really for people that can't work on their own stuff or have the money to pay a shop to fix it. Parts just wear out on these, and thats just the nature of a 20+ year old vehicle.