250 to 283/307 Swap

[JBX66]

Before you ask, I'm not asking for technical advice on how to actually do the swap. A friend of mine is offering a rebuilt 283 or 307 to put in my truck. Currently I have the 250 I6 with a single barrel Rochester carb, linked to a converted three-on-the-tree. Right now the 250 has (I believe) a rear main seal leak. Every time I drive it (which is not often since technically it isn't road legal) it leaks a large amount of oil. The 250 has somewhere along the lines of 91,000 miles. My friend likes to rebuild engines as a hobby and is willing to give me one of the two engines and help me put it in if I do some work for him (he's got a '91 Burb that he's retrofitting with a modified and carbureted 350, and needs someone to help him put it in). My question is, as I'm a soon-to-be-driving highschool student, what is the most economical option? Pull the 250, replace the seal and possibly rebuild the engine itself, or put a new engine in it? I need relatively good fuel economy (yes I know. "If you wanted good gas mileage, you shouldn't have bought a 60s truck"), say averaging 17 mpg or above, with more on the highway.