on the gasket should be sufficient to seal any air gaps. You could add an additional amount of RTV. Two things I would do to troubleshoot the source. While the plenum is off revisit your injector installations. Remove the rail and check each rail to plenum oring installation. Use Vaseline to ensure they are seated. When you install the plenum start your tightening sequence from the center outward. Also make sure the lower plenum bolt holes are clean and lightly lubricated prior to installing the plenum. Once the whole shebang is together and running perform a boost leak check with some soapy water around the plenum area if you still have a leak and can't isolate it.
