[rotorwrench]

One thing a person should keep in mind is that a bucking a hot steel rivet in effect is a forging process that closes the metals bond structure up a bit during the process. I would venture that they are not much softer than a grade 5 type bolt since both can be easily drilled with a high speed steel drill bit. They put enough of them in the frame gusset locations and for braces to strengthen the joints pretty well. You can use bolts, but bolts will loosen in service unless they fit the hole very tightly and even then, they should be periodically retorqued. Those old frames flex a lot more than modern ones.