If your going to drill the rivet the most accurate way is to use a drill fixture with hardened drill bushings to aline to the rivet head, guess work doesn't always do the job.
And as far as a grade 8 bolt. First there are bolts that are much stronger than a grade 8. Second, a frame will twist and move some and sooner or later loosen the bolts, unless you use loctite retaining compound, these are not large strong frames like in a Kenworth truck. Rivets were used because they are fast to use in the factory setting, and will not loosen and tighten way more than a bolt of equal size will as well as carry more load. On a bolt the treads reduce the diameter that is carring the load, where as a rivet there are no threads to do that.
