When it comes to Grade 9 and to a somewhat lesser extent Grade 8 for our cars, I'm more of the "just say no" position for the application. Note some of the things mentioned in the Fastenal stuff linked. The big deal to me is the loss in ductility which is the loss in elongation between the yield point and the fracture point, the bit about hydrogen embrittlement and increased susceptibility to fatigue failure are just the icing on the cake. For the most part that is the nature of high strength metals, as static strength goes up, elongation and fracture toughness goes down. I spent many a year dealing with materials professionally and lots of that was fastener related. If you want to do anything "special" with respect to fasteners, I would suggest that you use AN/MS/NAS hardware for most of your Triumph needs. Your basic AN stuff offers grip lengths in 1/8" increments as well as standard and thin washers so that you can adjust the grip length even further to make sure that you keep the threads out of any shear loads in shear type joints such as suspension pivots. Strength is between that of your Grade 5 and Grade 8 SAE fasteners. With much of the MS/NAS hardware, you get into more fasteners that are in the Grade 8 strength range and with several, you can get into grips that in 1/16" increments. Prices for AN hardware aren't all that bad. I can buy AN hardware from Aircraft Spruce for about the same price as getting SAE stuff (most grip length and fastener length increments there are 1/4" steps) from one of the local fastener places. Aircraft Spruce has a more limited selection of MS/NAS stuff, try Pegasus Racing and Coast Fabrication for a better selection of MS/NAS stuff, just expect to pay more for that type of stuff relative to the more readily available AN hardware.