Would not unnecessarily remove rivets, they're a pain to put back in. If I had to remove some I would rerivet them, not use bolts.
If you insist on using bolts get flanged frame bolts and nuts, I get them from the local big truck parts place, they usually have to order the small ones but have big ones on hand.
That crack is a serious problem, I'd suggest you sandblast around it and take it to a professional welder for repair.
I would not box the frame there, I'd consider a smallish plate welded inside over the crack once repaired, but a quality repair shouldn't need it.
Grigg
