The boot needs to have a 3/4" small end, 1-11/16" Large end and an overall length of 3-1/2" to 4".
I've tried some Ford Master cylinder boots, and they were too short. Darrin Wright on here found some Lockheed boots that worked, but had too large a small end. Squished up ok with a clamp though by the sound of it.
I'm having a look at a few options this week.