If you are just looking for gas mileage, just go with an AOD or T-5 tranny!!! Went the T-5 in my 56 and managed to get 27 MPG on my last trip to Columbus!!! Not kidding! RPM dropped from 3000 RPM @ 60 MPH to 1800 RPM @ 60 MPH with the T-5! The T-5 also made my car launch a LOT harder out of the hole as the FMX had a 2:97 to 1 first gear, while the T-5 has a 3:35 to 1 first gear.
This way you win at bot ends, more power out of the hole and LOTS more top end than the car can handle!!! (3400 RPM in 5th gear at 110 MPH!!!

) And I am getting better gas mileage than my neighbors 2007 scrub pickup!!!!
I love the smell of burning rubber in the morning!