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69hurstolds mentions, the old "printed" parts manuals from back in the day would typically list the bolt specs for a given part number. In some cases they didn't provide an actual factory replacement part number for bolts, and just assumed you replace with appropriate off the shelf hardware. Pretty much any substantial fasteners from suspension to engine components would be in the Grade 5-8 strength.
See sample here:
https://gbodyforum.com/threads/bolt-question.84825/#post-896940
The 80's were a fun time with transitioning from US standard to Metric fasteners in some places, but not all. When I need good fasteners I usually hit the local ACE Hardware, they have a full line of HILLMAN nuts/bolts, and about every type of clip, screw, pins, stainless, black, chrome, etc..you might need. What's best is they usually have them out in the aisle where you can browse the drawers instead of behind the counter where if you're not sure what you might need get's annoying when you have to depend on a clerk to get everything for you. In the latter case, you'll really want to just hand them your list with specs to save a lot of time.
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