TH400 to 700/200 transition

[musiqlova]

I have an 86 C10, GM Crate 305 (no mods), a built TH400 w/shift kit, and 3.08 gears. I'm getting around 13 MPG and I'd really like to increase that.

What modifications would I need to change from a 400 tranny to an overdrive tranny like a 700/4R or a 200? I'm sure i'd have to find/modify the driveshift right? How many MPG increase should I get with an OD tranny?

The tranny I have works great for me, but it's overkill for me. I have no intentions on racing (street or strip). The shift kit in it makes it shift REALLY hard. For a local deal, i'd be willing to swap a strip ready TH400 for a decent OD tranny.