what size bolts for spring hangers to replace rivets? and part #for shorter shock?

[dragn91]

I'm going to be lowering my truck next weekend so I'm trying to have everything I can lined up for me to do so. I'm going to be raising my stock spring shackle and hanger brackets 3-4 inches and drilling new holes on the frame I need to know what size the rivet hole is so I can have the bolts ready on hand as I do all my crap outside by myself so I'd like to have it ready to go. Also does anyone have a part number for a 3 or 4 inch shorter shock? My truck is a 1980 c20 thanks in advance.