Transmission interchangeability

[blarson at Zeiss.com]

Does anyone know for sure if a 1990 80 quattro 5 speed manual transmission is
interchangeable with a 1990 100 quattro 5 speed manual transmission?  It seems
like they should interchange if the final drive ratios are the same, but I have
a feeling that the 80 has 4.11 final drive, and the 100 has 3.89 final drive.
Could someone please confirm this?
Thanks,
Brian