The 1057 converter is perfect for racing or street rodding. The 1056 would be better for sled pulling and towing and all round driving. The 1057 is a smaller, lighter converter to help motor spool up.
The Jr kit is a great entry level upgrade. I would say its a great hold me over mod, but a full shift kit or performance built valve body is best if you are in the power game. For just a tuner (like a Diablo or Edge) and thats it, I think the Jr kit is perfect.