I don't have an 18 foot long HW but can provide this information from my 18 foot long Otca from 1936 which should be similar. The forward edge of the bow seat is at the 16th rib back from the bow. The back edge of the bow seat is at the 18th rib back from the bow. The forward quarter thwart is between the 19th and 20th rib back from the bow. The middle thwart is in the middle of the canoe (obviously). The rear quarter thwart is between the 19th and 20th rib forward from the stern. The forward edge of the stern seat is at the 12th rib forward from the stern. The back edge of the stern seat is between the 9th and 10th rib forward from the bow. Someone with an HW can probably provide more detailed information. Please reply here if you want more details. Thanks,
Benson