Boyd, I have not seen the floor, so my comments may be completely off-base. If the gaps or cracks are 1/2-inch wide, more or less, you might want to take up some boards from a back room or closet are and rip them (or whittle them, if the gaps are not long)to make suitable wedges that you could insert and Gorilla glue into the gaps between the floor boards. Jam enough wood into the gaps that some of the wedge is still a bit proud of the floor surface. When the glue sets, usually a day, sand the wedges flush with the floorboards, then finish the floor. If you make the large gaps smaller, then your sanding sealer and finishing material may be enought to fill in the remaining "smaller" crevices. Floor projects always have surprises, and require some clever innovation to achieve acceptable results. In that sense, your problem is not out of the ordinary. If you use filler wedges of the very same wood, it ought to expand and contract about the same as the floorboards around them. Without seeing your floor, I cannot imagine that you are having all this trouble with a solution to a problem that all of us have faced at one time or another. For a rustic floor, the use of wedges to fill the large crevices ought to be esthetically pleasing, and probably historically accurate. Let's see what the gang has to say about this approach!