Front crossmember rivets loose???

[Critter]

The swb 72 in my sig has had some steering issues since I got it a few weeks ago. I was told all the suspension components had recenetly been replaced (and visually that looks to be true) but it hadn't been alligned so it drove poorly. Recently I got around to checking the toe in/out with a tape measure and found that it was fairly correct. The problem lies when I put it in reverse, especially if I turn the wheels while in reverse. The passenger side wheel goes off at a crazy angle and no longer is tuning at the same angle as the driver side. It is so out of whack that the passenger tire will skid on asphalt because it is too far out of alignment. When I put the truck back in drive and move forward everything sort of falls back in line and it drives pretty decent in a straight line. Finally tonight a friend and I took turns crawling under the truck while the other moved the steering wheel. Turns out the FRONT CROSSMEMBER IS LOOSE! The front portion of the frame rails around where the steering box and idler arm are both able to move in relation to the crossmemeber that is riveted in place right there. The frame is straight, no rust and no apparent collision damage (my brother owned this truck in the 90s and it drove great, definitely no wrecks have occurred since then). It looks like the rivet holes have been enlarged giving a lot of slack for the frame and steering components to move around.

My question is this, am I screwing up if I tack weld the crossmember to the frame rails then remove the rivets so I can replace them with bolts? If I go through the trouble to weld it I don't really want to grind them away but will this make things too stiff and cause stress to collect elsewhere and create failure at another point on the frame? I appreciate any input on this. Has it happened to anybody else? I searched but couldn't find anything. I'm really bummed at the moment.