2 of the carrage bolts that hold the bed down on my 1985 c-10 are lose,since the top is a rounded head i dont know how to tighten them and my box slightly jumps around when i hit bumps. what can i use to tighten thme??????????????????

 

Sounds like the wood is no longer holding the bolt tight.
Brake fluid works well for freeing up corroded metal. Ya might try it.
Only other solution I can think of, is to hold the threaded end of the bolt with a pair of channel locks or vise grips, loosen the nut, cut the bolt above the nut and replace the bolt. If the hole in the wood is rotted out, that's another prob. ( Chevys used to be called 'Stove Bolts', due to the same style used on the old wood stoves. ) Stove bolt, carrage bolt, same same.