Nobody has mentioned yet one of the more common reasons for small block end seal leakage. Did you use the end rail gasket retaining/locating pins & is the intake drilled to receive them? Many aftermarket intakes don't have the receiver holes drilled in the end rails, so the intake sits on top of the pins and doesn't compress the end seals.

Personally, I do as others have mentioned and omit the end seals and use Right Stuff to seal the ends, or at least a premium silicon. I wipe the end rail areas with rubbing alcohol several times and dry thoroughly before applying sealant.