Dude! You're obsessed with making things complicated, aren't you ?
There are many benfits to going with the leg o mutton rig specified by the designer - not the least of which is the fact that IT WAS SPECIFIED BY THE DESIGNER who has worked out all of the complexities of the relationship between the center of effort and the center of lateral resistance, the size and position of the dagger board, and a few other critical things that make the difference between a well balanced boat and one that will kill you ( well, that may be a bit extreme...) In any case, if you're going to mess with the rig, you're going to have to either do a lot of research and reading about this stuff before you can make a semi-educated guess about sail size, mast position, etc. or you're going to have to consult a naval architect or someone with a lot of experience. Either way, the result will be a guess - educated, but still a guess - and you won't know if you've got it right until you're actually on the water sailing the thing.
According to Karl S. the leg o mutton (a.k.a the boomed sprit) rig offers an excellent compromise between the performance of a marconi rig and the simplicity of a sprit rig. That's why he likes it so much. The only drawback is the length of the mast - it won't fit in the boat for transport or storage like a sprit or lug rig would.
Build the boat as specified. Sail it for a season and then decide if you really need to mess around with it.
If you feel you really need a sloop rig (more strings to pull, right?) in a small plywood boat, check out the Pooduck Skiff at http://www.woodenboatstore.com/prodi...number=400-102