I don’t think this is supposed to be a gas vs. diesel discussion.
He’s had both and he knows the difference. That Hemi is more powerful than anything we were towing with 25 years ago. It’ll work.
The complicated part is selling your diesel for what it’s worth. Most buyers are going to need financing. The dealer can solve that problem for you, but you’re getting robbed, your truck is worth more than the gas truck he’s offering you.
If you’re able to sell your truck privately, do it then buy the longbed of your choice.
Avoid that “trade-down” at the dealer if you can.
Trying to find a long bed diesel to trade your short bed for would probably be better, simply because of the similar resale value.