Dodge LA to magnum heads

What casting head are you replacing? There are compression ratio difference based on the combustion chamber size. Also, if your old heads are early ones, along with your exhaust manifolds, you'll need to plug the air pump holes below the exhaust ports, or, you will leak exhaust. If you're working on a '64-6, you'll also need to grind some clearance on the driver side rear exhaust port for the factory manifold to bolt down.