A couple of comments. In the initial post, the statement "Still runs great, but I am a little anal about oil leaks", seems to indicate that you are not unhappy with the current performance, but would like more. If that is actually the case, intake, carb, headers and a rv style cam will give you a nice little boost. If you are serious about 300+hp out of a 250 it will cost as you will get into serious head work; 1.2hp per cube is expensive on virtually any motor.
Since changing intakes is mentioned, it must have the early style head. I thought the 80's had an integral cylinder head and if does not, it is likely some major work has been done to it over time.
Also, remember there are costs involved in swapping out the six to a small block or even a big block. You may need different motor mounts, fuel line, throttle linkage, exhaust system, accessory brackets and many more little items which all add up.
