With some exceptions, a stock bore and stroke L6 motor isn't going to have enough compression to limit boost, the head gasket, water jacket and lower end will limit the boost before this happens.
From what I've seen, high CR flathead engines are generally hot-rodded Ford V8s with aftermarket heads, or strokers - in which you would not want to run the 9:1 etc. they have.
Another factor is that if you reduce compression, the off-boost power really takes a dive, as does gas mileage.
I'm working on a side-valve (motorcycle) with about 6.5:1 static CR, and expect to run between 10-15 psi through it (Eaton M45).
