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...