I do not have specs and a suggestion. Last time I needed OP bolts I went to ACE hardware after testing with a few bolts I had on hand. This for a non-stock HV pump. I picked what I needed based on length. At least 3/8" thread engagement and better 1/2". I used washers and mocked up w/o washer to make sure there was no bottoming.

I used grade 8 though grade 5 should be fine.