For what it is worth... do what you think is safe. For me, I have overloaded every truck I have ever owned and have never broken anything. My 54 half ton has take many a load with no trouble. The trans and rear end may or may not be as heavy duty as the origninal. If you put a load in, just take it easy on the way home. If the rest of the truck is mechanically sound, you should have no trouble. If it feels wrong, then you may want to slow down or even unload some.
If my dad can put 7 full 5 gallon pails of compost in a Chrysler 300, you certainly should be able to load up an old Chevy. Good luck.
