I wish I cauld tell you what to buy, but let me say this about the B&M: I had a 2400 stall behind a 375 hp 350. It performed well for the most part, but I felt it slipped too much at higher rpms - inefficient. Sure, its supposed to slip coming off the line, but even at rpms above 3000, it never felt like the engine was directly connected to the wheels.
I had a local company build a converter for my BB, and it stalls to about 2200, but over 3000, it almost feels like a lockup converter.