I had missed all this about your wheel studs and nuts, but I've recently gone through much of what you're doing.
I have 70's era Appliance wheels that are now obsolete antiques and broke the 'shaft' part off of one nut. Well! Quite a big deal. It's a smaller O.D. than any other I've seen. I ultimately decided to modify the wheels to take a more-standard 60 degree tapered nut, but doing that makes it critical that I get longer studs. The old originals probably weren't long enough for these wheels anyway.
So while I'm doing that, I think I'll go up to 12mm x 1.5 size. I'll have to get the hub off. Haven't done that yet.
I got the studs yesterday, P/N CLP9037L for a Land Rover Freelander. They are supposed to be same knurl diameter as the Spitfire ones we have and they seem to be (but I didn't have my glasses on to read my vernier caliper). Rimmer Bros. in the UK has them. OEM and aftermarket at about 1/3 the price.
Here's an article about the change as applied to a Triumph:
http://auskellian.com/paul/links_files/upgrade_spitfire_wheel_studs.html Once I get the hub off, I think I'm home free.
Good luck!.... I have pictures and about 147 different dimensions, if you want any of that.