Worst case, perhaps best case, invest in the effort and tooling to set hot rivets. This is also what the shop manual recommends for frame repair.
There are several discussions on riveting on this forum and others, and some good YouTube videos out there too.
An easy option short of riveting Would be flanged head frame bolts and flanged self locking nuts as used on newer truck frames. These may not fit because of the large washer head?
Or in my opinion second best to the original rivets might be "Huck bolts" which are a sort of modern cold rivet/bolt requiring some expensive tools. A truck repair shop could help with this. This is primarily how new heavy truck frames are assembled.
