Hum. That's odd since I have the same one and no clearance issue in my 68 Cuda. The cap your referring to is probably what's called a "crab" style cap. (Because they look like crabs in a way). I've seen them made for other things but not a Mallory but I haven't looked for a Mallory either. If I run across one I'll post it here.
I just thought of something. Is your left motor mount good? I have my motor bolted down solid so it doesn't rise up any. If yours is rising up that may be why yours hits and mine don't.