You don't need to drill the cros-brace in order to use it as a seat belt anchorage. If you look at the above picture from the inside of the trunk, you will see that the bracket simply straddles the tube. A flat plate is then bolted to the bracket from the front side of the bulkhead and the seat belt anchor point bolt to the bracket and not the cross brace.
There is an odd irony that a replica (Kirkham) is more "original" in this respect than a CSX car.
However, I assume the change to thick-walled round tube was done for a reason and it may well be stronger than the original style square tube. I don't believe there will be any problem with mounting the shoulder harnesses to the cross brace tube. Bare in mind that the mounting is very close to one end and not in the middle where it would possibly bend.
With regard to the crotch strap, Kirkham use a "V" belt with 2 mounting points that bolt under each front seat mounting.
Paul