P/S pump bolts sheared off

[65chevysub]

While driving yesterday I lost my power steering. I looked and found my pump and cradle hanging loose; all the bolts had sheared off! I was driving normally so no hard use. Its been on since 1999. Had half of one bolt left and it has 6 hash marks on top. Don't grade 8 have only 3?. Any ideas why these may have broke off? Also, I'm pulling pump off today so need to know what size plugs I need to plug line ports in in gearbox until I get bolts drilled out. Thankd