Has the intake or intake faces been cut? The distributor might be bottoming out on the oil pump drive as opposed to the manifold.
The way to check this is to drop the distributor in without a gasket. It should sit down all the way without any gap. If it doesn't then you need to use a spacer.
The way to check this is to drop the distributor in without a gasket. It should sit down all the way without any gap. If it doesn't then you need to use a spacer.