Thanks for the replies guys. When I found out the trans came out of a drag car I wondered whether the gearing could have been changed to different ratios, or whether 6th had been removed, but 1st through 5th were the correct factory ratios (as listed in the link SuperB70 provided as well as this writeup:
http://www.camaros.net/forums/showthread.php?t=98387 ). As far as going back into 4th instead of 6th, the seller and I tried the 5-6 shift three or four times to confirm we were getting into the proper gear, so I'm pretty sure we were in what was supposed to be 6th. I agree though, we wouldn't have had the leverage we would have been used to if the trans was in the car, so I guess 5th->4th may have been a possiblity despite our efforts. I wasn't comfortable opening the case to look at the configuration.