The 4th gen 3.8 wasn't a 6 speed, it was just a 5 speed. The V8's were all SBC pattern. You could get an adapter plate to go from an SBC to the BOP I'm sure.

You also may want to check out the Tremec TKO 500 and 600. They are only a 5 speed, but are rated at either 500 or 600hp (hence the name) and Tremec has really opened up the market by allowing like 9 different shifter positions, plus several types of throw out bearing arrangements and even bellhousings. You tell them the app, and they will sell you a "kit" with alot of what's needed. I think the kits would be about $1700 or so.