Just remembered, I saw a '63 Chevy Biscayne? (Not an Impala) 2 door post with what appeared to be an early 30's Ford transverse leaf mount welded to a crossmember and the I Beam radius rods connnecting to the frame farther back.

The old Fords were LIGHT even with a huge V8. Seems to me going the I-beam/transverse mount single spring on a full bodied, fullsize 60's car would be asking for trouble?