[spacemanspif]

Price out a running cummins and then figure out how much gas you can buy with that. Then if the math doesn't look convincing enough, add the price of labor and all the extra bits for the engine swap. I think you are looking at around $15-20k for the project and that's a lot of gas. Once you factor in the the mpg differences and cost of fuel you will find that break even is pretty far down the road. Constantly driving across the country will help reach that point faster but it may still be many years.

On the other hand, the cummins should be a much better towing power plant that could make the cross country trips more enjoyable for you.

Strictly speaking, the money is reason not to...but if it's what you want then money doesn't matter as much.