Sorry, But the question begs, Do you have any manuals at all for your truck?
http://67-72chevytrucks.com/vboard/s...d.php?t=556578
In answer to your question yes you can replace rivets with bolts in the frame per the Chevrolet Chassis overhaul manual. The problem is that the bolts are not a direct replacement. The bolts need to be a very tight fit in a reamed hole vs a conventional bolt application clearance. This will then give you close to a rivets load capacity. So size up from the rivet and ream.