you can jump over to
http://www.60degreev6.com, and check out the transmission part of the website. Check what ones will bolt up to your 3.1. I know what you have is probably the 4t60e, the 4t65e may also bolt up. These are the ones out of 3.8 lumina's and impala's I believe, and I'm not for sure, but you may also be able to bolt up a getrag 262 if your looking for a manual. I'm not for sure on tranny's, you'll have to do a little homework, cause I'm probably wrong, someone correct me please. Also if you looking for fairly decent prices on tranny's, check out ebay, if you can do a local pickup sort of thing, you could fine a remanufactured tranny for a very reasonable price.