Martin is on to something ... Use the modern truck trailer combo for the horses but still restore Granpa's old farm truck they found under old hay at the back of the hay barn and that's what he drives to school, etc. Short of Jerry's recommendation about putting the sheet metal on a modern chassis and drive train, there isn't much (if anything) you could do realistically to my 3804 to make it capable of pulling the identified trailer in a safe and secure fashion in modern traffic and for longer hauls (i.e., it could move it empty around the farm yard but that's it.)
By the way, we have a family friend who, as a teenager, had to have dad take her horse to shows in an open U-Haul trailer because they could not afford to buy a horse trailer ...

Any of you who have experienced the whole horse show thing know how crushingly embarassing that must have been for a teenaged girl... (OP ... You could use that as a minor incident ... it really happened. Ooooooh, a possible romantic interest ....)
Ed makes a good point. Go to any ATHS show and you see lots of camper and toy haulers that are needle nosed Petes. Reo's, KW's etc ... but they are all air bagged to do that. If Grandpa was a rancher, it is not inconceivable that there might be an old Class 8 truck lurking in a barn or out on the fenceline, etc. People like Grandpa (and me, actually) tend to develop a real fondness and even a bond with a heavy truck they've spent a lot of wheel time in and had adventures with. Where I work (parttime -- I'm *supposed* to be retired

), my boss keeps his first road tractor -- a short-nosed Ford 9000 or whatever it is ... I looked at it long enough to see the word "FORD" and all the rust. I pretty much lost interest in it after that ...

). It is SCRAP METAL but any time I suggest adding it to the load to go to the scrap dealer, he kinda gets mad at me and subjects me to stories about it ...

. So I've quit bringing it up. I've developed a bond with the 359 Peterbilt I drive routinely. He bristles a bit when I call it *my* truck.

We local 'Bolters have a great friend, Dan, who is a retired trucker. He restored his last OTR truck (a 359 Pete. 359 Pete's have a VERY loyal and devoted following. I'm one!) into a camper hauler. It's beautiful. It is also VERY airbagged so as not to shake the camper apart.
Anyway, the point being that in your story, the kid leans on the fence gate with his elbows, his face cupped in his hands (like all farmers/ranchers do when they have big pondering to do) ... looks at his ginormous horse trailer ... looks at the 1-ton truck ...frowns .... looks at the horse trailer ... frowns harder... Then notices Grandpa's derelict 359 in the fenceline (in a direct sight line beyond his 1-ton) ... cocks one eyebrow up .... looks at the horsetrailer again .... jerks his head up and stands erect ... an idea forms .... the dog at his feet looks at him, then looks at the pete, too, murmurs an almost imperceptible disapproving whine and walks off .... (Dogs know better ...)
Still, thanks to the OP for giving us this opportunity to spend lots of fictional $$$ on possible builds for your character. This is a fun exercise.
So .... I think where we stand is that the kid is going to end up with the condo on wheels, a late model cowboy cadillac (my vote is for a crew-cabbed Ram 4500 with the 5.7 Cummins and the Aisin 6-speed) and a killer resto of an AD 1-ton. Either the family is rich or that kid is going to learn a LOT about finance ...

Thanks,
John