I have no personal experience with either of those so anything that I'm about to say is second hand. I've heard nothing good about NADP customer service after the sale. Everyone that has had one break that has posted on here has complained that they won't cover the breakage and basically says sorry about your luck, you'll have to buy another one. Suncoast seems to be a good transmission, but when comparing the price you can get the same or better for less money. Have you thought about DTT, Goerend, or Garmons? They are the big 3 on this forum and what everyone here has had the best luck with. I have DTT's full transmission and I love it!! I had several problems with the stock transmission and have spent what seems like hours on the phone with Stefan and Bill trying to fix it before I got the $$ to go all out. They can build you whatever you want to hold as much as you want. I would at least keep my options open and call some other places first. The 3 that I named above are not new to the business, and they all have built several transmissions for diesel drag trucks and diesel sled pullers with execellent success and have awesome customer service. Welcome to the Cummins Forum, sounds like you have a very nice truck, and you won't believe the difference when you get a built transmission, it won't even feel like the same truck!
Doug