I don't believe the TH425 was ever offered as a dual pattern trans, but I do know they can be modified to take the Chevy motor.
The problem with the 500 Cadillac is its limited potential. You can't make them much bigger using readily available parts, while the Chevy design can be bought as a crate motor up to 640 that I've seen.
The beauty of the Cadillac is the torque it makes in basically stock form. Looking for low-budget performance they are great. I have two of them sitting in my shop for future projects.
Rebel your tire spin with the 283 illustrates the challenge you face. The difficulty you already have connecting the power to the ground is going to get worse when you increase the power.
The physics just do not favor a pickup against the fast imports or Mustangs either. The weight bias means the pickup is going to struggle to get the same traction as these cars you want to race.
