Now that I have a later distributor with centrifugal and vacum advance capabilities, is there a way to modify the original Holley 4000 on my '54 Merc to get ported manifold vacuum to the distributor?? I'm hoping that someone has done this before by plugging certain passages and/or opening others to block the venturi vacuum and open the manifold vacuum passages.  Right now I'm using full manifold vacuum from the wiper port on the intake. I'd like to have the option of trying both sources. Thanks :  Bruce