GM 6.2 or 5.3

6.2L/8-spd without question. The 5.3L is not yet mated to the 8-spd. The 8-spd gearing is not quite as significant as 4-spd to 6, but the lower first gear and closer gear spacing combined with 20% more power will be very noticeable, especially whee towing. The cost? About $2k up front and ampg non towing. Towing fuel costs will be the same, possible a bit better with 6.2L given the extra power an gearing.

Now finding a 6.2L will be challenging. You may need to order one. You'll probably find more availability if you jump into the GMC Sierra Denali.