Engine upgrade or swap

Pretty much all of those are high torque low HP engines built to move land barges. None of them have real potential for HP without a good amount of work. With a good cam, intake, headers and some good tuning on a Holley the best I've heard on the 500 was around 400hp. Spend a lot more and you can get more HP but by then you have replaced pretty much everything but the block.
If you want to dig a bit try:
https://cadillacspeedshop.com/
Most of the parts work on all of the big Cad engines 368, 425, 472, 500 but none of them live long past about 4,500-5,000 RPMs without adding big money.