Is this a restore job or are you building a hot rod? If it's going to be a hot rod I'd ditch the wood and put metal in the whole car to brace it all up. Not and option if you are restoring it though.
I did replacement body wood for years as "Willys Wood" the best wood to use is poplar. It's easier to work with, won't crack as easily as oak or ash. It will hold a body nail well and your upholstery guy will like it (much easier to nail into). And yes it's a hardwood.
Pressure treated wood is for chevies (it keeps the termites at bay) but has little to no value as an anti-rot or moisture resistant wood. -It contains a pesticide of sorts and nothing more. Also, by it's very nature, pressure treated wood is choosen from very light and porous lumber which makes it even more prone to rotting away (but you won't have a termite problem until the moisture degrades the wood enough for it to loose all it's poisons) If you think that you can build the multidimensional/precise body supports in less then 5 hours and for less than the relatively inexpensive price of the reproductions then go for it. Just my humble opinion, open up that dusty purse and pop for the blocks and get it over with. (-they are oak/Hardwood)
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