: You don't want the slots any longer or a hard hit on a dock corner
: could crack the rail.

The solid Philippine mahogany 3/4" X 5/8" outwale with epoxied 4 oz fiberglass tape on the outside will have to be up to the task even with a 1/4" space of 3"...but, let's hope for the best.
My 7'7" S & G skiff with 3/8" X 5/8" Philippine mahogany outwale has held up well with fiberglass only on the stitched seams. BTW, it weighs only 14 lbs w/o oars. The Coot, however, has more than 4X the volume & weight EMPTY and so, is subject to much more stress. If something happens, that's why they make chainsaws, isn't it?...did I really write that?
This whole process: making the strongback & frames, ripping some very old redwood decking (nail holes and all), making bead & cove strips, stripping, epoxying & fiberglassing, designing & fabricating the mast step & CB trunk and now the outwales......a terrific learning experience. But the SANDING, oh, the SANDING.....
I'll figure out how to post some pictures when the wee beast is completed.

Harry