I ran into that exact same issue thirty years ago, when I installed my SW mechanical tach and only had a Moroso cable.

What I did was to go to a local machine shop, left them both pieces, and they machined an adapter out of a piece of brass. As far as the end that goes into the tach, I brazed the tach end of the cable with a big clump of brass, and then took a file and filed it to fit the keyway in the distributor.

How does it work? Well, the setup is still on there and is what I use, and has been since 1974. I guess you could say it worked out.