5 bolt valve cover leaks

First - the fabbed covers can be really be in terms of flatness. Second - the intake can interfere as noted above so check for interference where the covers go past the tops of the intake ports. Third - because there is no flange to help retain the gasket - I'd glue them to the covers with weather strip adhesive prior to installation on the engine, and yes - I'd only use cork for them. No RTV. Tighten evenly and moderately to avoid warping the covers.