Ive filled the bed to the top full of wettish sand one time... totally trashed the springs. Other than that ive hauled alot of hay in it usually around a ton to a ton and a half... steers real nice, but pray you dont have to stop, and be prepared to use the e-brake. As far as towing the worst ive done is TRY to pull my 77 F-100 4x4, w/ 460, and a spare 460, c-6, 205, d-44, and 9"............ i said try, cuz i made it about 20 miles and FRIED the tranny (FM-145) totally thrashed the main shaft. But i was kinda hoping it would... secretly i was hoping for a reason to do my drivetrain swap, which im currently immersed in.
BTW: I realize those things above mentioned were stupid and dangerous, but all done on back country roads (NO traffic) and i am no novice at doing stupid things, and i do not suggest anyone try it.... my truck was free, so i didnt really care what happened to it.