The 2004R is a 4 speed auto tranny made by GM. It was used in mostly BOP cars with v6 engines. Most had a BOP bellhousing, but some had a universal that would fit either the SBC bellhousing or the BOP. This tranny was most famously used in the mid-80s Buick Grand Nationals and T-types, both of which had the screaming 3.8 turbo V6. Check to see if the GNs or T-types had floor shifters. Most aftermarket shifters won't work with an automatic with 4 forward gears.

Let me know what you find, as I'm gonna do a similar deal with a 2004R behind a Poncho 400 in a LeMans.

Good luck

Chip

Preaching the Hot Rod Gospel according to the 4-stroke apostles:

Suck, Squish, Fire and Fumes