How much do you haul?

[Tofuball]

I'm new here, and I've been reading tons of threads, I see a lot of REALLY nice trucks, but not much more then some parts or an engine in many of the beds (Maybe I just need to read more threads)

I've got a 75 GMC 2500 3/4 with the TH350 and NP203 - I was wondering how much I can throw in the bed (not on a trailer) before I create an unsafe situation - and what mods I could do to let me carry more.

Mostly I intend to haul gravel, scrap iron, and 80lb bags of cement, so I'd like to make as few "safe" trips as possible.

Thanks!