Still trying to find out a number for you. I talked to the guy with the shorty plugs on his car and he said he used NGK and now has Autolite, both shorty plugs.
http://www.carcraft.com/techarticles/64378/ is an article on plugs and will give you some general info on them. Also type up shorty spark plugs on the internet you will get some hits and they might be of help.
Remember too, that the threaded section of plugs come in different lengths, from .375 to .75, do you have after market heads, I know my edelbrock victor JR's use the .75 with the gasket but I was able to get away with not using the shorty plugs so can't help with the numbers.