How much difference will a new converter make

"just a purple shaft" and "a 750 double pumper"... Sounds like you may have a larger purple shaft...Which one? If the tires stick, and if the cam comes on after 2500 (like the MP .484 and .509s) then a stall convertor will help you. With 4.10s, I dont think you need THAT high a stall tho, regardless of cam. I'm a big fan of "use the lowest stall that get's you into the torque curve" when it's a street car that races once in a a while.