318 Dis-assemble & Assembled 1st time

What you are finding is not the piston's fault. It's more than likely the block machining from the factory. When an engine is rebuilt, it is normal for the shop to mill a little off the decks to make them "flat". But if you start from the factory machining, you may be way tall to begin with. This is not a dimension most shops check during a stock type rebuild. The term "square decking" means the deck surfaces are not only flat, but set to blueprint height and parallel to the crankshaft's centerline. As for pistons, if you have stock replacements already, they will work fine. But you won't get as much compression. Keith Black has a couple of modern hypereutectic pistons that are designed to address the lack of compression. I usually run them at "0" deck and use the gasket and chamber size to set the compression ratio. On the magnum swap, I would also do this. Aside from tapping and plugging (really not needed..the head has no port in it. So the head IS the plug). Whne you are assembling, install the heads using an old set of gaskets. Lay the intake on the shortblock, and closely examine the intake port flanges and head flanges. Put a light behind the engine, and site down the gaps from the front. If you see light, or the intake doesnt "settle" in and seal off the lower edge, you need to mill the intake or the intake flange of the heads a little to have it fit properly. Leaks at the bottom opf the ports are usually the result of poor intake fitment. If you had to pry the intake to start bolts, or had issues starting the bolts with your fingers, you had intake fitment problems.