I don't know if you can even get a vacuum advance for the Mallory that you have.
If you do get an original distributor, get it rebushed (Advanced Distributors), and have Jeff talk you through whether you need a port or manifold vacuum dashpot. If you're running the SU carbs with the integrated float bowl, I think those were manifold advance originally, and the separate float bowls ran on port advance. All that means is which side of the throttle butterfly that your dashpot vacuum hose plugs into. It does make a difference, though.