if it's a 55-59 Chevy, 3 speed or 4, it came with a 3.90, with an OD trans the original ratio would have been 4.11 - it's the GMCs that might have had the 3.07 .... and yes, the 60's rears were different, although still 3.90 ratio

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature we find it attached to the rest of the world" John Muir
"When we tug a single thing on an old truck, we find it falls off" me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot