New tow vehicle...6.4 hemi or Diesel?

My old 95 3500 Cummins I bought new. Now 225,000 mi , old 12 valve AT 4:10 gears gets 18, and 13 pulling. If it had the usual 3:54 it would get 5 mph more. I pulling nothing but good size goosenecks down to my 18 ft car trailer, yes in OD! But it also has none of the modern smog BS either.
I was pulling with 454 1 T Chevies decades ago and getting 9 back then!!!! lol