Cass @ Doctor Diff in Montana would be the frist choice
Strange , Mark Williams will also build a diff to spec , Mark Williams will have a superior product but neither would break in a street car .
I have grabbed a diff out of a 70s ford truck with a clutch sure grip & the ratio I wanted for less than $100 then ordered axles for around $500 & housing ends for around $100 & brgs for $100 & then get the housing narrowed for about $275