I used to think that the crew cab longbed was a goofy look for SRW trucks, but they have grown on me a lot and I really dig them now. The ultimate is still a proper extended cab longbed, but only Ford is making that now a days. If you wanna talk about goofy, the megacab DRW trucks look super goofy to me. The proportions of the cab/bed are wrong in my eyes, but a lot of people like em and I've seen some really clean builds so they can work.
As far as 2500 vs 3500 registration stuff, I only know my own state, but I always assumed that states plated/registered based off the GVWR, not the "model". I'm also surprised to hear that GM has different frames for their 2500 and 3500 trucks, that strikes me as odd. One thing to consider is that with Ford/GM trying to be "sneaky" with >10k GVWR 2500s, it's very likely that states start to take note and update their registration to counter this.
End of the day, I don't really understand the difference in going from a 6.4' bed to a 6.75' bed. If those few extra inches are critical, I guess by all means, but it seems like a silly argument to me. Also, idk what your new trailer is, but I'd strongly consider looking into a dually, even if it's not necessary. For one, they are way more stable, which is nice. For two, you can legally tow way more, and you have an extra set of tires in the event of a potential blow out, which is nice. Plus, man they look good!