One of the best ways to find out for sure is to take the bolts off of just one side, and measure them (then go out and buy ones that are however many inches longer as the body lift). That is what I did for the 1" body lift I just installed yesterday on my 1985 K30 Dually. That being said, the factory bolts on my K30 pick-up where 5" for the radiator support, and the main cab bolts where 3 1/2" long. I am not sure if a K-5 uses the same size bolts, but with the way Chevy does things, I would not be surprised if they where. Go for Grade #8. They are cheap enough, so why not. I bought all the Grade #8 bolts for my truck for ony $16. The Grade #8 bolts for your K-5 will probabaly be only about $25 or so.
