What's the best transmission shift kit?
A "shift kit" requires trans removal and tear down. In case you didn't know.
If it's a 5sp LLY
If it's a 6sp you can get a TransGoJr kit and you only have to mess with the valvebody
But I see we're talking about a 5sp
"Best" option?
If you're looking to sustain substantial horsepower increase, which is technically anything over 100hp, your best option is a "built transmission" which is going to include a better torque converter, more durable frictions and steels and a myriad of other possible parts
If you call the number given above, don't say "shift kit" in any sentence, Mike doesn't deserve that, and know you're talking to arguably the best diesel transmission guy in the business