fin placement is a hard thing to nail down for a few reasons.
1. COE of the sails. This simple for IOMs because of the 3 standard rigs.
2. CLR of the keel.
3. Efficiency of the keel.

Any of the fin mast box combos from SailsEtc should work as long as you use a keel fin from them as well.

for side stays, you want to have several attachment points, so that you can adjust mast bend. Lester Gilbert have an infinitely variable chain plate on his Italiko.
http://www.onemetre.net/OtherTopics/Italiko/Italiko.htm

Feel free to send as many pics as you want (the bigger the better), I will be happy to post them.

-Dan

Thanks for making your design available. I’m a first time builder and have constructed the hull and deck. I’m now attempting to build the mast box, fin and rudder. I have a few questions in regards to the placement of the mast. It appears from the plans to be quite far infront of the keel. I was looking at some of the prebuilt fin mast box combos available form sailsect and they seem to be about 10 20 mm infront of the leading edge of the keel. As well where sould I place the side stays in regards to the mast position? If you would like I can send some photos of my build so far.