Grade 8 hardware would be more than enough for the seat. Grade 5 would work just as well. It isn't in a stress area that would require a higher grade bolt. Once the bracket is bolted securely to the floor pan, the entire pan is utilized for the stress area.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...