James, as you probably know, wood floors expand and contract with changes in humidity. The winter will normally be the driest time inside your house. This will normally be somewhere around 6% EMC. The EMC--Equilibrium Moisture Content--is the amount of water you wood wants to hold and will, given time at these conditions constantly for a while.
For comparison, our house runs 12-14% in the summer with everything wide open and, I just checked the chart and it's 9%, inside now but this will drop a little as the house warms up.
The wood will slowly react to these changes, and remember that it will see the changes in EMC from all of it's sides. If you could totally waterproof the top of the wood, is would still move because of the moisture changing on the woods bottom face and sides. The wood will move about 1% across it's face for a 3% change in EMC, less along it's length.
Since the changes are more across the width but are a percentage of the width, a wider board will move more in actual dimension than a narrower board. You probably can see that. One other thing that you probably know is that you can not fill the spaces now without future problems. If you put any kind of filler in the spaces, the wood will try to crush it and the filler will pop out or the wood will be crushed. If the wood is crushed, this will be permanent and the gap will be wider during the next contraction cycle. The pegs will tend to limit the movement but will also experience this crushing over time.
So the solution for a finish is one that will move with the movement of the wood. It also has to adhere to the wood. The finishing/refinishing is best done in the middle of the summer when the wood is as big as it's going to be. My choice of finish material would be an oil based polyurethane. You do want to really work to get sanding dust up from between the boards as well as to get the dust that's on the walls and everywhere else after sanding.