Anyone heard of Boss Hog torque converters? Is this gonna work for my setup?

I've heard of them (quite a few years ago) but never used one myself so I can't comment. The guy who installed your trans/conv probably didn't seat the conv properly and/or didn't torque the flex plate/conv bolts correctly. The stall may change based on your components and the range may also be representative of the differences between SBs & BBs. It would be a good move to call the mbfr with your specs (usually done before ordering) and see what their recommendations are. GL.