A Transmission decoding can be somewhat tricky, and getting all the info can be difficult. We checked with our go-to guy, Ken Casey at Burt Chevrolet, and as we expected, he didn't let us down.
Ken found that this transmission came from a '92 B-car which in the Chevrolet line is an Impala or Caprice with an L05 TBI 350. The car was a four-door with 3.08:1 rearend gears. Your recycling yard man at least had the year right. Unfortunately, it's not from the IROC-Z. However, this is a great transmission core to hop up. It will accept a nice shift kit, and you should replace the Second gear servo cover and piston with pieces from a Corvette. This increases the clapping force of the Second and Fourth gear band for better shifts and durability. TCI Transmissions offers its Trans-Scat Valve Body Kit (PN 376000), designed to deliver a solid shift suitable for towing or a competition-quality gear shift. With most models, you are able to manually shift and hold the transmission in First gear until you shift into a higher gear. The kit vastly improves the Second and Third gear shifts, improves performance, and helps extend clutch and band life. While you're at it, be sure to pick up the heavy-duty servo kit (PN 376003), which will add 300 pounds of force to the band application for extra durability.
We hope this info helps. We know it may have been a little bit of bad news about the trans, but if it's a solid core, you'll have a great trans with the components listed above. By the way, Ken Casey said this is the exact transmission he sells over the counter for people wanting to install overdrives and he has very good luck with it. Good luck and keep on reading.
If you have technical questions for Kevin McClelland, send him an e-mail at firstname.lastname@example.org.