I have a 1990 auto. D250. I need to pull about 12,000 pounds, on top of that, the trailer will weight about 4,000 pounds. It's on flat road. I know it depends on how good the weight is distributed on the trailer. But take a typical trailer.... bad idea or good idea?

Around 16,000 pounds. The book says it can only tow like 6,000 pounds and with my 1990 F250 3/4 ton, it could haul a lot more than that.

 

I pulled a 6k pound trailer and a 12,5k pound two ton truck with my 92 auto. 31's 3.55's and bone stock. Trans was rebuilt with a shift kit, I kept it out of OD. You should be fine, unless your going a long long ways, and will suck some fuel. If a trooper pulls you over, you'll be in it deep......

 

My D350, all stock(only upgrade is 4" exhaust and KN Filter), set up as a dump truck, weighs in at 8k lbs. Ive loaded 4 ton in the dump and pull 4k lb trailer. Like CUMMINS said, keep it out of OD.