Northdm,
Your question is a good one. Grade 8 bolts should be fine in the applications you discussed. To back up my view I did a bit of research and found this in an article from "Stock Car Racing":
Grade 8 bolts are safe to use on basically every application on the racecar with the exception of wheel studs and highly loaded engine fasteners such as rod bolts, main bearing cap bolts or flywheel bolts.I would add the following however. All fasteners, especially Grade 8 fasteners, need very flat and parallel mounting surfaces. If the surface that the bolt head face bears against isn't flat it side loads the head which can cause the head of the bolt to pop off! The reason I mention this is I will often use allen head or the 12 point type fasteners like those made by ARP
http://www.arp-bolts.com/Tech/Tech.html which have a smaller head diameter and built in washer faces. Some bolts even have special spherical faces with matching washers to ensure the bolt head doesn't see any side loading from non-parallel mounting surfaces.
Also the other very important factor is correctly torqueing the fasteners. Dry vs. zinc coated vs. lubricated: they all have different torque values based on the size of the bolt and its thread type. Here is an excellent article that discusses these factors and includes a torque chart.
http://www.zerofast.com/torque.htmI hope this helps.
John
