The mahogany outwales on my guide are about like this drawing.
Finished Dimensions: A=1", B=11/16", C=5/16", D=7/8", E=9/16", F=5/16"
The height dimensions (A, B &C) taper a bit in the last 10" or so of each end and wind up in the neighborhood of 13/16" for A, 9/16" for B and 3/16" for C at their extreme ends. Width dimensions (D, E, and F) don't taper. Outboard corners are slightly rounded, inside corners are square. You need to be sure that the little protrusion (dimensions C&F) fits neatly over the exposed top of the planks and canvas and up against the rib backs, so measure on your boat before cutting the notch in the stock. I'd also cut the whole thing a little tall and sand or plane it down to the proper height as part of the installation or test fitting process. Old town steam bends the last three feet or so of their Guide outwales into a curve to help them make the sheer line's upward sweep at the ends. There isn't an awful lot of it on a Guide, but mahogany is fairly brittle and it might be quite a battle with straight stock.
You can also buy pre-cut, pre-bent outwales from Old Town. If you order them from a dealer who has a shipment of boats coming in soon, they'll often just stick them in the truck with the canoes and charge you little or no shipping fee.