F7R/P head bolts

With the club shop no longer supplying head bolts for the Volvo engine which also fit the f7 engines and I've been to every nut and bolt place local to me and numerous calls to other suppliers I've failed to find high tensile versions :(

So here's my next question, how much abuse does everyone think the stock spec head bolts can take?

What about checking out stud and nut conversions from other makes?
If you want to experiment I have loads of spare bolts... I'm also guessing the extreme values will be affecting the threads in the block more than the bolts.

I know the ARP stud kit for the F4R will work but it's not a cheap option