I'm not sure that frame rivets were cold rivets. They sure look like hot rivets. I am in the trucking business and we repair and rebuild our own trailers. We use aluminum rivets, but I have set 3/8 hot rivets with our riveter. It is like an air chisel with a .498 shank size. We also use Huck bolts. This is a little more expensive. A puller costs around $3000. There are also some ribbed shank siderail to crossmember bolts that are available at trailer companies. The whole idea is to use something that fills up the whole hole. A bolt only squeezes two pieces of metal together. If you are not worried about retaining everything original looking, the ribbed shank bolts would work fine. They are available from Trailmobile or Wabash or any dealer stocking dry freight van parts. I would weld a patch over the cracked hole, drill it 3/8, put in a rib shanked bolt and it will probably last another 50 years.