Just to add a bit here, the standard headers are fine for all but the most tuned of bikes, so the 40mm diameter should be fine. as far a the lengths go, a very rough rule of thumb is that longer makes the engine more ... "bottom endy" and shorter moves things to be mor "top endy". 2 into 1 should work, HRC found it to be better for the SP2 in its last season so....
Other than that, copy one that works/is proven if possible. otherwise you could have a gorgeous exhaust system but a motor that is hard to tune or runs like crap. Exhaust tuners/builders are pros. not without good reason. You wouldn't let a gynacologist give you brain surgery would you???
Chris