Neat job to shorten the insulator without cracking the whole thing! Watch out for overheating although because of the diminished surface area. But according to a one of my old books AC 87 could be a bit softer than Champion D21. Say could, as different old lists translate heat ranges differently. I did rummage through my little stock of NOS plugs as I remembered that I have a pair of AC’s. Unfortunately they turned out to be AC 84 wich is three steps cooler, so to say.
I suppose that you have a radiator with neck, otherwise with a tunnel radiator it’s possible to raise the radiator. This by making an offset upper bracket and substitute the lower screw with a rubber cushion that the radiator rests on. By doing this I even got place for proper plug caps.
By the way, I don’t think there are any other plugs shorter than the the AC ones. Can’t remember seeing any.
Best regards and good luck!