You said that a diesel would be better for mileage but a big block for power. I'm assuming a diesel would be better for towing, correct? (more torque to get going?).
The cummins by FAR will give the best towing power you could ever want, BUT AT WHAT COST?
And that being what will it cost you to convert a gas truck frame to a diesel powered vehicle?
You would be best to find a salvage diesel truck forthechangeove so you have ALL THE PARTS you will need
A big block gas will give you good towing power lot less cost to build and when time to sell probably about the same resale as the diesel converted
I have had my cummins for 11 years but driven it since it was new. It is an absolute towing machine since I had the pump tweeked, billet converter disturbution hitch, love the towing abilities, BUT, if you really don't plan on LOTS OF OVER ROAD MILES or FREQUENT TOWING HEAVY LOADS, just do not see the need for a diesel for most people.