I cannot see any benefit of the splines, they do not hold any pull from the bolt, just prevent a threaded insert from turning in its bore while doing up the bolt. You´d typically go for a helicoil but I guess the mangled bore is too large for that. So instead I´d fabricate a bush with threads inside and out after finding a tap for the oversize F106 bore. With that lipped nut all the force from bolt load goes into the small cross section at the lip/head. So no wonder you can snap it and pull the knurled part into the frame tie - not great.
An even easier way is to line ream the frame tie to get a nice parallel bore out of the mangled hole. Then you can do a decent measure of the bore and fabricate a nice threaded and lipped ss bush with the 1/2 " thread and loctite it into the frame tie with type 638 or 648. Still all load will go into the lip cross section but there will be a bit more material after reaming to oversize first. No need for knurling,the Loctite cares for locking the bush safely while tightening the bolt.
Vic
Vic , The bit that snapped on mine was the other end, Standard it is 1/2" stud with a nut brazed on the end, But the new one I bought was Stainless !, The stud was one type of stainless and the nut was another,
It had been on there for a long time and some sort of reaction had happened, I put photos of it on the Forum,
I was a mechanic all my working life, Not known for over tightening things, But I had put a spring washer under the head, And one day as I was about to go for a ride, I looked down and saw the nut part was on the piss,
Soon as I touched it, Off it came. Cheers Bill.