rivets

Good Day, I have a newly acquired 93' Prado that has been in the shop since 11/9, just a week after purchase, and I'm dying to drive the thing. its currently flare-less and I was really trying to get these darn things in the mail before I pickup the truck. I'm having a heck of a time finding...

I would like to fabricate and replace the rear crossmember on this FJ45. It is a little beat up but not corroded. Not full understanding how the frame rivets work and if I were to grind them off…what would be the process in riveting the new crossmember back on? Can it be bolted or is riveting...