i took a quick look at CL and found this, there were a few more trannys up for sale in the tri state area, but all were for a n/a motor when i was looking, this one is a hd trans, and just across the GWB.
HD trans should be much better for a top swap build like you have. would need to get the pcm programmed to match the gears tho.
2000 BUICK REGAL SUPERCHARGED PARTS