the standard bow seat takes the place of a thwart so generally not needed -  maybe go ahead and install the sliding seat and watch the hull while someone is sitting on it, sliding it forward and back, and see if the hull is flexing a lot or too much - if so , then I'd say put the thwart in

a thwart is a handy grab bar for lifting the boat, so you might want to install a front thwart if it works better for that then the seat you will be installing, and it would give you something to lash gear to, which a sliding seat may not be good for.   Only real issue with adding a thwart is would it make it harder to load your gear in the boat.

you could post your question over on paddling.net  Rob Sarges, the builder, watches that and would likely give you a better opinion ( I think he goes by "openboater" or something like that)

I finally found a nice boat at a great price on Craig's List. It is a 16'3" Vermont Canoe Encore tandem  :dankk2in kevlar. I plan to replace the rear thwart with a kneeling twart. Would it be overkill to move the old rear thwart to the front? I have seen 16' foot boats with and without a bow twart. I will be car topping the boat on the highway alot so I was assuming the extra thwart would help the boat. I am also planning to add a sliding bow seat so the weight of the boat is increasing - I will not do it if it will not help. Thanks for any input.