tryna get close to 400 hp...Wanna work out my 973 casting 318 heads

The only way to get that kind of hp out of a 318 is going to be to turn a lot of RPM (smaller displacement means more rpm for same horsepower).

To make that work you are going to need a lot of cam. To make that work you are going to need a lot of gear (and a high stall converter if its an automatic). And when you get all that going, it is going to be very short on low end torque, which is where all the "fun" in "fun to drive" comes from. It is not going to be very streetable by almost anyone's definition.

If you're going to put better pistons and deck the block, a 340/360 head would work a lot better than if you tried it with stock .100 in the hole pistons.

Your "good" castings wouldn't really offer any advantages expense-wise over something you find: 302s, etc. if you're looking to add springs, valves, valve job and milling. And for all that money you could probably get some LA-X or EQ magnum heads that will flow a ton more and already have hardened seat inserts (and then you would have cylinder heads for the bigger motor you will one day upgrade to). These heads also have much smaller than stock combustion chambers to build compression, and they are a favorable wedge style, rather than open.

ALL the power comes from the cylinder heads (and to a lesser degree the camshaft), that's the last place to skimp when trying to make horsepower.