When it comes to a material to use for porches, the new flooring offered in speed grown pine will not compare to the old Long Leaf Pine but do not think it will not also deteriorate if you are not caring for it right. You can use the 3/4 Long leaf Pine, there really was not an interior and exterior version so much as a grade based on the growth lines per inch which should be the more the merrier and the tighter they are the better. You can not get a good species for that application now, not even the kiln dried and treated will last as long as the old Long Leaf Pine, especially if you keep it saturated with oil or protected so as to not swell from rain.
The wood types for porch floors vary but are not different inside to outside on the really old houses. If you want to upgrade to the best, you’ll need to find the thicker sizes that ware also used, preferably Cypress but it is more than double the cost in the 1 1/8″ class or the LLP in the thicker class of 1- 1 1/4″ which is also more expensive. That is why is was seldom found on porches after the 1920’s in Texas. Anything but those two woods and it will not survive long, but if it is Pine or any wood but the rainforest woods, and it is let sit in sun and rain without care, it will deteriorate.
We stopped making steps outdoors with the stringers from wood for the same reason. They must be oiled every year or two normally but in a drought, in the sun of Texas, it needs to be done at least once a year where you add thinner to Tung OIl and soak the wood for twenty minutes, keeping it wet then wait twenty minutes without applying more, just letting it soak it up, then wiping off and excess left so it does not get sticky on the surface.
Hope that helps for those who want to add many years to the life of their porches and stairs. We have switched to fabricating steel angle iron stringers instead of using wood for outdoors due to the lack of care issues. Thanks for the inspiration to write this ditty for the people who may need it for care with their tongue and groove wood porches too.