I have an HEI distributer in my 67 400. It doesn't hit the firewall however it does give me another issue to deal with. My PCV valve hose has to go in at a wierd angle due to the vacuum advance being in the way which causes it to not seat properly which I think is part of the problem I have in my post (crappy carb?). But my issue may not be shared by everyone with an HEI.
Mike