Engine swapping is worth it if
A) you enjoy doing the swap
B) you have the parts lying around
C) you plan to drive a lot of miles and want the higher mileage
If C, it takes a lot of miles, and you'll never, ever get nearly the mileage of a decent $500 commuter car. And the $500 car may even turn out to be more reliable than a 30-year-old truck. So if you are concerned with the cost of driving, just buy a Saturn. It's hard to argue with 40MPG from a <$1000 car if you're concerned with economics.
If you enjoy swapping motors, or you wanna learn a new skill, by all means...we will be glad to help ya learn.
If you have the parts lying around, that works too!
But this isn't something that should be agonized over. Pick something and run with it! Remember, given how often 30-year-old trucks like to break, you'll have good opportunities to upgrade things later on if you change your mind...