I actually think I would drill the rivets and pop the outer plate before I would weld in new pieces. If the metal is rusty anyway you will have a hard time welding to it and expecting it to be strong. It is also likely (although I'm not really sure) that the frame is heat treated and welding to it would cause it to be weaker than if you left the rust alone. You have to remember that welding is the cheapest way of joining two pieces together. So if they could have Chevy would have welded at the factory....I think! It's been a little while since I was under mine and really looked, but I believe the rivets are solid and grinding the head off and hammering them out with a punch doesn't seam that unthinkable!
I run a fabrication shop, so my perspective may be a little different, but if I went the the trouble to pull the outer plates off I would more than likely just remake them! Possibly have them galvanized!
Just my $.02 worth.
