As long as you put that 3800 sheet metal on a modern Diesel powered chassis with power steering, power disc brakes, and 2 ton + suspension, you'll have rig that your readers with any sort of mechanical knowledge would find acceptable. Pulling that horse limousine with a stock-frame 1 ton pickup from the 1950's will be approximately as believable as having Snoopy's Sopwith Camel win a dogfight with an F-15!
I'd also suggest that you attend a few equestrian events in your area and take a look at what some of the horsey folks are using as tow rigs for similar trailers. I think you'll find that even suggesting that a 70 year old truck, regardless of modifications, would be up to the task would require a reader to take a deep dive into Fantasyland.
Jerry
