The stall speed will not affect the motor at idle, that should not be the problem. However I do agree that something may be fishy with the trans. Since its a B&M tranny I figured you bought it new and in that case it should be fine. Did you buy it used? Is the torque converter used? Something definatly sounds fishy. The converter could be way to tight I suppose, as in its not allowing the proper amount of slippage at idle. Do you have a stock converter you could try by chance? It would rule out an issue with the converter. I know those things arent cheap, I'd hate to buy a new one and it not fix the problem. Where did you buy the trans and converter from?
The correct stall speed for you will depend on a number of things. A link to your camshaft would help. Also the weight of the car will be a big factor here. Remember when shopping for a torque converter that two different converters with the same stall speed will act different even in the same car. The amount of torque produced by your engine will also factor in to your stall speed.