I converted to full manifold vacuum on my 68 L30 327 many years ago, but I had to make modifications to support the change. Initial static timing, vacuum advance canister, mechanical advance springs & limiter - these are the changes that I can think of off the top of my head. Just making sure that your distributor "rework" includes these modifications. You can't just solely change the vacuum source. If you already know all this, please accept my apologies.
There is a great article out there by John Hinckley that was published in Corvette Enthusiast entitled "Timing And Vacuum Advance 101". If you google search that title you should find the article.