300 Inline 6, Rebuild or Swap?
I don't know whether a 300 six can be made to perform like a V8, but since I just moved from an `89 inline with stick to a `93 351 automatic, I'll say this much.
The six had far more low end power. I got about what you report as milage 12-14, but now I get 9-12. I've added new high capacity cats, Flowmaster exhaust and an air induction system, but the milage stays the same. The ability to get off the line fast is fun, but listening to the trans shift down on a medium grade is irritating when I know my 300 cube would have tugged up the thing with no problem. I'm thinking that if what you are looking for is pull power, either stay with the 300-6 or go to a 460-8. Mileage isn't going to improve in any event. I really do get some pretty decent performance out of the 351 and it is fun listening to it, but it is a truck after all and I'll always miss the 4.9L that got trashed in my driveway by a runaway Mustang.