If there is room (hard to tell from the picture) the easiest option might be to shorten the nipple on the vac advance (It doesn't need to be that long) and put a vacuum elbow on it. Shorten the nipple side of the elbow if necessary. Might be the view of the picture but it looks like the bump out on the cap for the primary voltage wires will hit on the firewall if you try to reindex the distributor counterclockwise. Doing it this way you will still be able to adjust the vac advance. You might have to loosen the cap and rotate the distributor to get at it.
Not much room for an hei in there. There are other smaller cap distributor options also. Good luck with your project.