Actually, you only need grade 5 bolts for mounting, IIRC!
The race car I still own has everything solidly welded directly to the frame. One that I built in years past was an early TVR Grantura, which had a tubing frame with tubing lighter than current exhaust pipe tubing, and a fiberglass body that was simply bonded onto the frame tubes. Of course they rotted inside the places where the fiberglass wrapped around them. First I had to cut the body off the frame, then rebuild the bad bits of chassis, and then figure out a way to put the body back on as well as attach a roll bar to such lightweight tubing. I ended up solving both problems by welding triangular plates across the corners of the frame and sandwiching the new floor sections I'd created out of fiberglass (I itched for weeks) between the roll bar and the chassis plates. Great little car - weighed 1700 lbs. I'd love to still have it - it would make an unequalled (well, except for an Atom) platform for an LNF drive train!
Most challenging cars I was ever involved with from the point of view of roll bar/cage creation was a Lotus Elite (1960s, not the later car of the same name) which bolted suspension sub frames to a monocoque made completely out of fiberglass (they tended to tear out of the body as the cars got older), and an early Marcos, which ran a Ford 1600 motor and had a chassis made out of plywood!
