Push come to shove and the x member needs to move (and I would re-install it as close as possible) Just knock out the rivets, then replace them with some grade 8 hardware and steel self locking nuts (with threadlocker)

Isn't this hobby Fuuun?? lol

 

what Gary said..... drill/drive/grind them out and replace with same sized (or one over) grade 8 bolt with a locking nut and threadl ock -red. Take out the cross member and get on with it. The shackle will never know it's a bolt or rivet if you tighten it properly and probably not then...

I never replaced the cross member but I did install a trailer hitch at the rear that does the same thing...

later
John

 

You're on the right track. just grind off the rivets and slide the crossmember to the end of the frame and bolt it in. Replace the rivet you removed from the spring hanger with a grade 8 bolt. Build the 1" square tube frame as outlined in the link you posted and mount that to the bottom of the frame rails.

You might need to trim about 1/4" from the bottom inner edge of the frame rails to get the top of the tank to fit between the rails.