my point was that the Gr5 bolt will stretch more for a given torque than a Gr8. this is actually good because that stretch is what applies the clamping load to the bellhousing. Now if the transmission/engine assembly is loaded in compression WRT each other those bolts will relax slightly. But because they stretched more, they still will retain some clamping force. This is actually good for longevity so long as the clamping force is sufficient. Stresses that vary over a narrow positive range are better for longevity - in fact you can determine a range using steel where lifespan is essentially infinite (unlike aluminum, but that's a whole 'nother lecture) if that range is narrow enough, below plastic deformation, and always positive. Stresses that periodically go to zero or worse yet reverse (tension to compression and back - but you won't see that in a bolt application) are far worse, as is adding a shear component. You can get more clamping force from a Gr8 bolt without getting into the range where you risk plastic deformation, but you may run the risk of ripping the threads out of the cast iron block to achieve that torque as cast iron is not as strong as steel. If more clamping is desired I'm guessing the design engineers would have either upsized the bolts and stuck with Gr5 or else used steel thread inserts in the block to make the steel/iron interface effectively larger and used Gr8. Since steel inserts are not common except as repairs, I'm going with rule of thumb I'd probably use Gr5 (or 8.8 which is roughly the metric equivalent)
Of course the right answer for any specific instance is to look at the head markings of the old bolts that come out, and match those if possible. It is possible to replace a Gr5 bolt with a Gr8 one in a bolt and nut application if you use the higher torque spec for the Gr8 bolt, but I'm wary of trying to over-bulletproof a tapped hole in a softer metal by upgrading the bolts.
just my 2 centavos...
