If I were doing it, I would do one of the following depending on what would give you the best angle for the rear stays while keeping the rear hoop at the desired distance behind the driver's seat:
1. Fold a spreader plate over the cross member, making sure that the flat part on top is large enough for the hoop tubing. A gas torch with a rosebud tip is your friend when making the plate conform to the cross member. To keep the tech inspectors happy, make the spreader plate much larger than the minimum size and well welded to the cross member.
2. Build a plinth (box) behind the cross member and weld the tubing to the top of it. Again, large and heavy to keep tech happy.
3. Same as #2, but in front of the cross member.
In both #2 and #3, having the top of the box extend over the top of the cross member would make me smile.
It all comes down to which one gives you the best combination of rear stay angle and distance behind the driver's seat.
"I don't suffer from insanity, I enjoy every minute of it!"
IOE winner in the Super Snipe -- Buttonwillow 2012
IOE winner in Super Snipe v2.0 -- Buttonwillow 2016
"Every Super Snipe in Lemons has won an IOE!"