Hi Scotty,
I second what Zack said - finish the interior before canvassing. And re “varnish or poly”, use a quality marine spar varnish with UV shields. It is more flexible than polyurethane, the UV blocking technology protects the wood, its easier to work with later on, and it looks much better in my opinion. My personal favorite is Epifanes, but other brands include Petit, Interlux, and TotalBoat. With high solids content and UV shields, their thickness builds quickly through multiple coats, and they provide great protection against water and sun.
I don’t oil the interior but rather start with 50/50 varnish and thinner and apply coats as per the manufacturer recommendations. After some coats of varnish, however, I do coat the exterior of the canoe with thinned boiled linseed oil or varnish or tung oil. This should be well cured before canvassing.
After spending considerable time varnishing you don’t want to damage your varnish when canvassing so be careful. If you canvas right-side-up, a moving blanket inside the canoe will protect the varnish.
If the advice is to varnish before canvassing because varnish may harm the canvas filler, then what about re-varnishing later on? Impossible? No, because by the time you might need refresher coats years later, the filler should be very well cured. In my experience, re-varnishing a canoe in subsequent years hasn’t caused any filler/paint problems.
Hope this helps,
Michael