seeing as that you are in yellow pine country that is what I would go with. It is denser than white pine though a little more forgiving than w/oak. if your going to use oak then you may as well use a steel or aluminum block. all the newer gms use a steel shim betwee the box and frame.
if you use wood over size the hole a hair so that it allows for shrinkage and won't vind on the bolt and split
ron