4 speed auto into '65 Barracuda?

The basic answer is to look for an A-500/42RH out of an early 90's Dakota, van or truck with the 3.9. The OD transmission is wider where it passes through the rear "crossmember" in the unibody, so this area of the chassis needs to be modified. The drop out section and rear motor mount need to be modified as well. Nothing here is super cosmic, a local hot rod or race car fabrication shop should get it done for $500 or so. You'll need to convert to the later style slip yoke driveshaft, and have it shortened. You'll either need a later universal shifter, or a special adapter cable set if you want to keep the original shifter.

Honestly, with 2.94 gears, you have a pretty street friendly setup now. The 4th gear would have the engine lugging below 65 MPH. I don't think it would improve fuel economy much. As it is, you should be getting 20-22 MPG, which is probably about as good as it will get.