First you have to figure out what you have. We have already established that you DO NOT have 1.5 ton running gear.

Lets figure out what you have and then worry about what you can safely stop.

My 1.5 ton snatches my 8000 lbs boat around like a matchbox, but anything above 45 and its not fun to stop even with surge disc brakes on the 3 axle trailer. My 85 suburban with hydroboost disc brakes will stop it on a dime (Okay, maybe a 50 cent piece).

You can bolt a 4100 cab down on a 1/2 ton chassis if you want. Then you will have a plate that says you have a 11000 lbs GVW weight rating, but you you don't, get the idea. Strange things happened to these trucks over the years. If a farmer in Nebraska had a rust free farm truck that he sold for $500 to some guy that hauled it down to Alabama, that took that nice cab and put it in place of his rusted out cab on his 3/4 work truck, but left the emblems on the side... You get the idea.

Email a few of us those pics from underneath and lets see what you have. I will help, I'm sure Grigg will, too.

Safe payload on your flatbed equals your rating.
1 ton = 2000lbs
1.5 ton = 3000lbs
2 ton = 4000lbs