Bob, my 181 came with the small diameter flywheel and it was tapped for a pressure plate. It also was Blanchard ground and ready for a clutch in a road application. The only modification from a car wheel I could see was the entire thing was painted in epoxy type black paint to prevent corrosion in the boat. It is a 1988 engine if that matters. A counterbore is correct to clear the clutch disc dampening hub but the one I have measures only 5 7/8 inches diameter. I checked a disc at 9 1/2 inches and guess what?, about 1/2 of the friction surface would be hanging into the counterbore you describe.
Hate to be the bearer of bad news but I think you need to get another flywheel.
