If one can not find gcwr, I've for many years before gcwr became a semi to know number, used the GRAWR x 2 for max trailer on that rig. and times 3 for max total of the truck and trailer. Power then vs today was way the heck less. A BB in 25/35 series truck had all of maybe 200-250HP with mid 300 lbft of torque, Std motor was a 300 I6 with 100-120hp max. Base motors today have more power than the BIG motor of yore!
This rule of thumb IMHO still works today. This is basically the largest trailer behind a vehicle before the tail wags the dog. If you want to go faster, you need more HP! no matter how it gets to the rear wheels.
Marty