water neck

The magnum neck is alot smaller and the bolt spacing is different. The Thermostat is also different. LA water neck will not work on the M1 intake for the LA engine.

Maybe you are overtightening it. Try it again but block sand the bottom of the T Stat housing first, clean it off, new gasket and kiss it with some Black or Gray RTV. Tighten it snug but don't get too excited and over tighten it splitting the gasket. Snug and a 1/4 turn more will do the trick! Let it set and I bet it will be good.

Ohh and drain the water down into the intake so no water gets on anything while it is drying.