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.

[morethanstock66]

If you want good gas mileage, out of those three, the 250 is the best option. It is also a really good engine-strong, lots of torque, ect. As you say, you won't get awesome gas mileage in a 60s truck, but IMO 250 is the best option. You can build up I6s to put out power also, later on down the road- offenhauser, Clifford, ect. I had a 250/sm420 with 456s in my '67 for 5 years.. And I drove that set up in high school also lol.

[geezer#99]

X2 on keeping the six.
Don't assume it's the rear main leaking all that oil.
Power wash the motor so it's clean enough to eat off then find the oil leak. Could be just the valve cover gasket.
Do you have a working pcv system?

[morethanstock66]

Rear main is usually the last in line to check. Valve cover, drain plug seal, oil pan gasket, oil pressure sending unit fittings are known to leak, ect. Clean it like Geezer says, run it for a bit and look, if it leaks a lot like you say, should be able to tell.