Linda, I have found that the best way to pull apart nailed structures is to cut the nails. Apply just enough prying to get a cut off saw (saws-all) in the gap and cut the nails. You then go back and remove the heads later (a must). The older floors I have done are tongue and grove. The nails were driven in the floor side of the grove at an angel. And didn't pass through two boards. However they also had sub floors. The only non-sub floor I have seen is old barn flooring, 2x6 T&G nailed straight down. Either way cutting the nails will do the least damage to the wood. Its a slow process, and people with more money than time will tell you it isn't worth it, But I think it is. There is nothing like breathing new life into that old wood.