The best RWD bellhousing to use on the Buick 3.8 V6 is one from a '94-'95 2.2L 4 cyl Chevy S10 that has the T5 manual tranny (the T5 was only used on the 2.2 in '94-'95...the '96 up 2.2's went to a weaker New Process NV-1500 that has an integral bellhousing). This bellhousing has the same engine bolt pattern as the Buick 3.8 V6, and also has a Ford T5 tranny bolt pattern on it that mounts the T5 in a straight up position (not the 17 degree tilted position that Chevy Camaro's use, which is such a problem for swaps in most RWD vehicles). This bellhousing also has the proper provision for a drivers side starter, which is what the Buick 3.8 engine block has when used in a RWD configuration (all FWD & RWD Buick 3.8 blocks have the starter mounted on the same side of the block).
By the way, the best T5 tranny to use on this bellhousing is one from a '90-'93 Ford V8 Mustang, since they have the proper 7.18" long input shaft that fits up lenghthwise for the pilot shaft to fit into the pilot bearing within the Buick 3.8 V6's crankshaft (though you will have to bore the Buick's pilot bushing out to 0.668" ID in order to match the Ford's input shaft size...the std GM pilot bushing is 0.59" ID). The reason to only use the '90-'93 Ford V8 T5 is that they are a WC T5 that is rated at 300 ft-lbs of torque (the L67 supercharged Buick 3.8 V6 is rated at 280 ft-lbs, so this T5 will hold up to it); whereas, the '85-'89 Ford V8 T5's are only rated at 265 ft-lbs of torque. Also, take note that the later '94 up Ford T5's have a longer input shaft that is 7.85" long, which is too long to fit up in this application.
If you want an even stronger manual tranny, an aftermarket Tremec 3550 5 spd with the Ford bolt pattern will bolt-up to the above mentioned bellhousing as well.
Hope this helps you out!