Bolt grades - seatbelts

["Dr L. Kevin McNelis" <kmcnelis@nmsu.edu>]

OK, I am a-fixin to put in a 4 point seat harness (mostly to look cool 
while I'm wearing it, hopefully it will never have to be used!).  The 
lap belt uses the original seatbelt anchors.  But I have to drill and 
bolt the shoulder staps.  I'm gonna make plates for the backside 
(underside), but should I use grade 5 or grade 8 bolts for the shoulder 
straps?

Thanks as always, Kevin