3spd to 4spd OD trans

The A500 (42RH) or A518 (46RH) are derivatives of the 904 and 727 respectively. It's the OD section that is larger than the 904/727. With either tranny you will need to do some floor surgery to make room for the OD portion of the tranny.

FWIW, A buddy of mine has an A518 behind a hot 340 six-pak in his 67 Cuda with 4.10 gears in the rear end, he gets over 20mpg on the hiway. I get 21-22 mpg with a hot 360 and an A833OD with 3.55 gears on the hiway and 17-18 around town.