For 600 bucks, I don't think you can get that great of a shift kit. At least none I know of. If you're going to bother with spray, might as well get a full nitrous kit with all the good accessories like a rpm switch, safety switch, bottle heater, purge valve, etc. Probably should also save up and get a tune for the nitrous. Just my opinion.
As for the 4.10's, I feel you should get them one way or another. You'll like the 4.10's with and without spray.