Alec,
There is a link to a "plan" in PDF on the main ec12.com page. That will show you exactly how an EC12 is set up/rigged.
You can see the size of the main and jib by following the "rules" link on the main ec12.com page.
Sounds like a great boat. Our boat is a tank test model from 1962 whereas yours is from 1958. There is only an inch difference in length, ours being 59 inches long, but yours is quite a bit heavier. Ours carries a maximum of 19 lbs of lead and weighs about 24.5 lbs with that ballast.
http://www.ec12.com/History.htm
The building site(s) that Skip told you about should help a lot in construction of your boat.
We had a club member a few years ago who raced a boat that was about the same length as the EC12, but was about 26 lbs. He used an EC12 suit of sails. His boat was a few inches deeper in draft. It was a 6 meter and the EC12 rig drove the boat very well. He was somewhat competitive in light wind and easily was faster when the wind came up. I just mention this in case you are considering using an EC12 rig. Seems like it might work well for you.
Scott