Stuart, the pine that I think you are considering--really called white wood and the variety range is considerable--is really soft. I think that you would have a lot of problems with that. If you can get an assured source of a good hard pine, like Southern Yellow Pine, that would be more than acceptable, in my view. SYP looks great with a coat or 2 of an oil based poly on it. I would suggest that you would be a lot better off with a layer of rosin paper under the new wood and if you can get a router and run tongue and groove joints on your new boards, it will be a LOT better. The T&G limits the gap dimensions that are wide open. You only nail at an angle through the tongue side of each board and the groove holds the other side. You want to let your new wood acclimate where you will be laying it for a month or so before you lay your new floor down.