Cordell, I have used the 2500HD with the 6.0 and with the 8.1L Vortec's.
The 6.0 with the 4.1 gears will pull the boat although will be out of over drive on hills. Overall, this engine /gear combo will give lower economy than the 8.1 with the 3.73 axle ratio.
THe 8.1L that I am currently using is a torque monster. It likes to be loaded and seems to continue to make power on demand. Itow alot from OH to FL & NC going through mountains with 6% + grades (I75) and from TN to AshvilleNC. I am up to 26K miles since December in towing miles logged (28 to 42 foot boats & Stacks of trailers up to 14K lbs in total stack weight).
overall, the 8.1L with the 3.73 gears is a awesome tow vehicle with the best economy loaded & Unloaded. It is really hard to compare the 7.4 to the 8.1.
Bottom line, go with the cubic inches.
If you are getting serious about a vehicle, we are ordering (at this point) 3 of the GMC Pro-Grade trucks (2 3500 and 1 2500HD) all with the 8.1L engines. These truck will have the small GMC Pro-Grade offshore racing logo's on them. Let me know and we can get you into the group promotional pricing through GMC Powertrain- Vortec engines. Studebaker GMC (Scott Studebaker) in Richmond, IN is the Pro-Grade administrator for handling the GM Powertrain (Vortec)discounts. The GMC Pro-grade program does include diesel applications as well although does not include the Vortec Powertrain division contribution.
If you prefer Chevy's ... we are setting up Jeff Layton (Lynn Layton Chevrotlet) - a Formula owner - for the Chevy Vortec program. The certificates come directly from GM and must be signed by an officer of GM.
PS: I have no monetary gain from this program other than enjoying the same discounts. Although I am supporting GM in their marine sponsorship program to even include trailer discounts with new trucks.
This is also a great way to let GMC know we all appreciate their support in Offshore racing.