John,
I'm shocked!!!
Greg; This is not a problem at all! The 3x3 square tubing that supports your roll bar is a perfect place to mount the shoulder straps for a 4 / 6 point seat belt system.
This square tube IS major cross brace for the rear of the frame. Simply crawl under the car and you sill see it. You can drill holes all the way through it and use long bolts with STEEL lock nuts to attach.
BUT, always a but, if you want to be SCCA and NHRA legal, put a 3" 3/16's angle piece of steel in place running parallel with the stock 3x3. You need to trim just little off both ends to get it to fit tight. Then simply weld this piece in place, then mount the straps anywhere you want through this piece!
It is really easy, for once a cheap modification, and you DO NOT HAVE TO REMOVE ANYTHING TO WELD IT IN PLACE!
I don't know where the pictures are of Cpt. Eddy's build that I did but if you can find them I have perfect pictures of this piece in place In fact it was one of the bolts I put in that I forgot to tighten up
that the Captain was complaining about! (More pictures.)
If this isn't plain enough please feel free to call me at 231-869-4608 EST
DV
PS
If I remember right I weld this in this piece leaving about a 1/2" gap between the 3x3 and the new piece. It allows you to mount your traps at a "legal" down angle from the shoulders.