what eng tranny
 

57 f100 short wide eng room empty I want it to be a dd so mileage is important I have an early 70s 390 c6 thinking about other options 4.6 aod ?:-X19

As much as I really love my FE engines they`re not the ones for any real fuel economy so you`d be much further ahead with the 4.6 and the AOD. I remember the 223`s were fairly respectable on fuel mileage, at least in cars. You`ll need to change the rear end gearing as well, a Ford 9'' with about a 3.25-1 or 3.00 to 1 would certainly help out alot.

A carburated 302 is not a bad way to go, but as Fomoko said, you would have to change the rear ratio for any real milage!