Choosing a stall speed.

I have a 70 Dart 340.Comp. cam 256 int.--268 exhaust Part # 20-221-3. 3.55 gears 727 trans -LD 340 intake with thermoquad carb. I had the front bushing in the trans replaced and the place I had it done replaced the converter .
at the same time. I believe it was stock from a 360.Not sure why at the time but since then the car has really bad low end pulling power.From a dead stop it doesn"t pull well but has lots of power midrange and top end.I just want some advice on a good quality,reasonably priced converter.