a couple of comparison points.....
Randy, all of the planes listed below have been .049 powered, so my point from this list is that the .061 will be more than enough if you can keep the little Shoestring around 14-16 oz's (if I remember the 18 ounce figure is for the electric version of that plane)
Spider- 240+ square inches, fairly thick flying wing, 10 ozs-hauls
GLH-200 square inches, fairly thick airfoil, 16 ozs--- hauls butt
Caliente- 165 squares, thin airfoil, 12 ozs--- really hauls butt
I have heard that that kit is a great one. You oughtta consider modifying it to look like one of the super modified Shoestrings that are flying at Reno right now like Nemesis(I think it is Shoestring based...
Get it done before SMALL, I would really like to see it. I love the Shoestring!