Absolutely no idea. I have one and only one experience with stealth heads and it’s on @Turbo440Dart Chris’ car. I was there when it was tuned on the dyno but have no first hand knowledge of the timing curve it left with. My guess would be they are possibly a bit better than stock 915/906 chambers but it’s a guess at best. What does the strap on your spark plugs look like at your current wot timing?