In the Winisk and the Quetico, a couple of other J. Winters' designs. I usually fixed the seats in, and quite a bit back to, I think around 60 inches from the bow to the front of the seat. The bows of those two canoes were quite narrow, so having the seats that far back provided ample leg room for most people. The Dumoine and the Yukon would be far more forgiving, given the generous and buxom dimensions, and could probably handle a fixed seat quite a bit more bow forward.
Given your fine wood working skills, a sliding seat is a great idea!