I had an 85 w250 that the PO added Timbren to. Not sure of the rating.

Rode very rough empty or loaded. Hated it.

My 92 d250 with a higher gvw rides much better.

I'm not a big fan of just beefing up the rear for towing, you're much better off getting the tongue weight right and using a weight distribution hitch. You get a better ride and Gain stability. Plus you can swap the hitch to a different vehicle easily.