I think it's just because of the name. It's different, unique.
There are MANY engines out there that have the basic requirements to be made into a racing motor. The difference is that they don't have to parts availability that Chevy, Ford, and Mopar engines do. But these days, everybody's looking for an "edge". Something to make their car stand-out at the show, and at the corner parking lot.
Hell, perfect example would be a recent event that was held at the local dragstrip, Englishtown, Raceway Park. There were a BUNCH of real nice cars there but one car got a rather dis-proportionate amount of attention. And it was only because it had an engine that was unique for the car. It was a ratty old Caprice that had been run through the mill as both a police car, and then as a taxi. The car was just as you might expect seeing it right out of a junk yard. So why was there such a crowd around this car? Why were so many people gawking at it instead of the brand-new H2 Hummer 30ft. away, or the customized Viper 30ft in another direction? Because it had a Big-Block 454 shoe-horned into it, that's why.
People are attracted to the odd, and that's what putting a Cadillac engine in a hot rod is, ODD.
I know quite a few socut owners who dropped a cad under the hood. IH motors have many of the same qualities as the Cad, High nickel blocks, ungodly low RPM torque, an longevity. the biggest down fall of these 'Truck" motors is weight most IH engines weigh around 800lbs. Thats alot of weight for a Small Block!
ok ok, i suppose i should chime in. my 500 is going into my scout. i am running a T-19 wide box 4 speed behind it, so i had to crack off for the billet flywheel. i had a custom clutch made, 11 7/8" chevy clutch housing, 12" ford clutch, semi-truck springs, and forks from a tractor of some sort. its gonna be ungoddly stiff, should hold all that touque well, if not i will just get an organic clutch. Im using the radiator from a 73 eldo, 4 row of course. like i said, motor is out of a 76, so i had to get the rear sump and and pickup tube as well. im in the process of getting my sandersons, and in a few months a edlebrock intake and i don't remember what sort of cam. i don't think i picked the motor cause of the cad name, i picked it cause there is no replacement for displacement, and i really enjoy the fact that i can ask some guy with his HUGE chevy or ford what motor they have, and they will smirk and say ' a 454/460 ' and i will just snicker and say, man, thats tiny! and watch my feet as their jaws hit the floor. the 500+ lbft of torque won't hurt either