Chili.
I have a '89 Ford van with the 351 cu in. engine and a C-6 transmission. It tows very well. It is a E150 with a upgraded suspension. My old van,'69 E100 came with 302 / 5.0 L engine and didn't have enough HP to get out of it's own way. I changed it to a 351 with a C-6 and it made a major difference in both pulling power and mileage. I sold the '69 van with over 300,000 miles on it and the guy I sold it to is still using it today to haul vintage cars.
Aside from towing dirtbikes I also use it to haul horses and equipment. It hauls my 4,400 Lb. Bobcat plus trailer with little effort.
I highly recommend the C-6 tranny. The electronic transmissions are not very towing freindly and are VERY expensive to repair.
Just my $ .02