I found out that if you go with the GM aftermarket Tremec
GM Aftermarket T56
This swap will be the simplest and least expensive. It will
bolt right up to your bellhousing, use your stock clutch setup,
stock fork, pilot bearing, throwout bearing, etc. This
transmission utilizes a 27 spline slip yoke, which is the same as a
TH350 yoke.
You’ll need
mechanical speedometer gear adapter
26 spline clutch disk
to modify your crossmember
shorten your driveshaft
From here http://5speedtransmissions.com/t56_early_gm.html