Ok. This is just my opinion.
I would think it would a little heavy for a 250. You are going to be over your rear axle capacity and payload rating and might be stressing some components, especially when towing the 3800# boat .
I hate to see you pass up that Snow River if its clean and has no delamination. Is a newer 350 out of the question ? Could you buy the SR and store it till you're ready for a more beefy truck ?
This is why i feel the way i do:
As an example , I had an Arctic Fox 860 on a GMC SB and that being over 4000# was too heavy for a SRW 250. The weight plus the extra height of the basement model slideout camper was too much and did not feel safe, crosswinds were killing me . The rear axle was sitting at 6800# .
My truck was new and the AF was leaking , so i got rid of the camper and kept the truck. I made the mistake not buying a 350.
Since then i moved down to a non slide Lance 845 with 6200# on the rear axle plus a lower non basement profile and it rides fine with my mods on my 2500HD.
Don't forget you want to tow a boat of 3800# .
You might have to get into a hitch extension to clear the camper overhang plus the extra tongue weight .
IMO, i think its too much for your truck . I tow trailers behind my tc all the time and 3800 is a good load with 4000# in the trucks bed.
Hopefully, others will comment and i hope my post doesnt crash your dream. Please understand, i want you to be safe .
Bob