We towed for many years with a 91 E150 conversion, 351w, 4.10's, and basically a 3/4 ton spring pack, class IV with bars. Pulled a 70 mustang in a 20' enclosed. It was enough, but never impressive. Got about 8-10mpg. A club wagon with a 460 back then would have been a much better choice.
When we traded for a 26' enclosed and started towing a charger or coronet, the van was outmatched. Switched to crew cab duallies, then to cummins trucks. Will never switch back to gas for towing.