Why do I keep blowing head gaskets?

We have a blown bbc at w.o.t. with 12lbs of boost the compression is 15.5 to 1
It runs on 100 LL av gas with a quart of toluene per 5 gals.

From the research we've done
100 LL av gas will support up to 15 to 1

The added toluene we put in just bumps up the octane a bit and slows the burn.

The OP should be fine with straight 100 LL av gas. (do your own research to make your decision.)

Also it's typically only about 1 dollar per gal. More then 91 here in AZ.
We recently had this Engine apart for a refresh and it was perfect inside.
No signs of detonation
We even reused the rod bearings because they looked like new after 5 years of running.

Just Our Experience...