The reason those hangers are riveted is to absolutely prevent any movement between the hangers and the frame rails. As a rivet is installed ("bucked" is the correct term) it expands in diameter to completely fill the hole its installed into.

I'd suggest using a slightly larger bolt than the rivet hole, one with a precision-ground shoulder, and reaming the holes for an exact fit. These bolts usually have an Allen head, and they're Grade 8 strength or above. Properly torquing the nuts, and using red Loctite on the threads should assure they don't get loose.
Jerry

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