My friend had an 89 W150 with a 318. it was a short bed, std. cab. We hauled an open , steel trailer with cars, other, bigger trucks, etc, all while the bed was loaded with parts, engine hoist and even a 55 Hemi once. The truck LOVED it. Best towing little truck I've ever seen.
I'm not positive but I think the truck had 3:55 gears in it too.

My Dad once towed a full-size Ford 4WD truck on a heavy equipment trailer from Las Vegas, NV to Mansfield, OH with the little blue '84 D150 shown in the picture, and had no trouble at all. It has a (mildly built) slant 6, highway gears, 4 speed manual, 4th is OD. That's my D350 SRW longbed crew next to it, quite a bit longer...