I have these concerns in adding the two seats at the back thwart and center thwart (yoke) positions— space, center of gravity, bow-to-stern balance and esthetics. Before doing this I would consider old fashion boat custions for the kinds to sit or knee on or the folding wooden seats that are positioned on the floor of the canoe.
Replacement seats are available and can be cut to length and bolted in as you suggest, but adding two seats to a 17′ canoe is not recommended. I would consult with a Lincoln canoe dealer/retailer.
We tripped with our two kids, dog, and packs throught those (4-14 year) age peroids using a 17′ canoe. I always liked the fact that the kids were in the bottom of the canoe rather than higher up when the weather turned bad or the dog leaped to shore 6′ before the canoe arrived.