The car drives like a 240 with a quieter more torquey engine. Nothing wrong with a turbo motor, and you may get the same mileage with some additional power, but with the taller gears the 300 na engine makes all the difference in driving the 240 on the highway.

If you didn't have a good na motor then getting a good turbo might make sense but since you have a good na motor I encourage you to put it in there.

I have done two 300 conversions of 240s both using the entire 300 ddrive train. Both made the lethargic and buzzy 240 into a quiet and somewhat more powerful car. The mileage is only slightly impacted with the 346 diff. Using the 307 diff the mileage is equal to a good running 240 but taking off is a bit of a chore expecially on a hill.

Its an especially sweet motor with a proper 300 flywheel....but they are only slightly more common than the 123 five speed.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.