If you have an older house and find a 3/4″ or thicker softwood plank subfloor by all means nail the new strip floor to this solid softwood. Just be sure to renail the old subfloor back to the joists with spiral standard nails that are at least 3″ long (or about 3 times the thickness of the subfloor). The original old nails in these floors have by now come loose, and this is the only chance to get at this subfloor. You can use screws to refasten the subfloor, but not drywall screws, use wood screws (about 2.5 the thickness of the subfloor).
I personally use 3″ spiral (ardox) framing nails, and drive at least two nails in every board, in every joist. I use a pneumatic framing nailer when I take this step before installing a new hardwood floor. But I am careful to set the pressure so the nails heads end up slightly proud of the surface. I go back over those rows after the magazine is empty and pound these nails flush with the surface with my 20 oz. decking hammer. This does two things: it assures that the nails will not be over pressured and set deeply into the wood, thus damaging the strength of the subfloor. Also the pounding releases most of the dust and debris that may be caught under the joist. This way my new floors are always squeak free.
Hopefully these old subfloor boards won’t be too much wider that 8″ otherwise they will have, in time, become warped. It will be difficult to flatten 10-12 ” wide subfloor boards, and you may have to sand off the humps to get a decent nailing surface. Don’t sand the subfloor any thinner than 3/4″. If you find a really bumpy subfloor, renail it anyway and then add 1/2″ plywood to even out the surface. This is particularly important with prefinished strip flooring, the smoother the subfloor, the smoother the finished floor will be. In fact, in most old houses with softwood planked subfloors, I don’t even think about installing a prefinished strip floor. Prefinished is very unforgiving material and is best laid in newer more level homes on plywood subfloors. That way it will look more like the sample you saw in the show room, not the bumpy floors that you see in some houses.
Read my free article on the Wrong Way Floor in this site. This article also deals with the direction of the new floor. Always install a strip floor in an opposing direction to the subfloor boards. The ideal plank subfloor will be laid on a diagonal to the joists and will allow the hardwood floor to be laid crossing (perpendicular to) the floor joists. You will now have a doubly strong floor. Otherwise if you want to lay the finished floor in the same direction as the planked subfloor, you must lay a buffer layer of at least 1/4″ or thicker plywood.