Vacuum limit on Mercruiser/ small block Chevy

[FishStretcher]

I have been listening to Bushwacker's tutelage on cruise vacuum setting for the Mercruiser/ small block Chevy.

So I was wondering two things:

How long can you run Wide Open Throttle before you have to observe the 7-9" vacuum limit?

What is the failure mode, and can we do something about it? I hear exhaust valves. But can this be addressed with better valves alone, and then is WOT all the time possible? Or is it valves and seats and manifolds and pistons...

Finally, I want to see if there is a knock sensor on my engine. If so, then I have a nice trick for a power adder.