Overdrive trans or change rearend/gears

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If you decide to go with a T5 look for a Mustang "World Class" (brand name) gearbox, and a Sonoma tail shaft to put the shifter in the right place. The Camaro/Firebird/'Stang T5 shifter is designed for bucket seats and puts the shifter to the rear of the tail shaft. IIRC, I don't think I had to change my clutch plate, but did have to cut 1/4" off of the trans input shaft to keep it from bottoming out in the flywheel. This was behind a 292 I6, so the 350 may be even easier. My T5 is a .83 OD, and the engine went from "scream" at 60 MPH, to 2800 RPM at 65 MPH. The T5 is quite, shifts great, and weighs less than your 3 speed.