what made you "realize" that? do you have some documentation that GM used a pickling process? it's kind of rare to find other than surface rust on the frame, but I don't think that has anything to do with 'pickling' ...... what you're doing blasting the frame is removing the usual surface rust, carry on, and paint it or not as you choose, most folks paint them, but some swear most years frames weren't painted, including me
Bill