Lou,
Treated wood is great for many applications, but I'm not sure it's  necessary 
or a good thing if you are going to be encasing it in fiberglass  anyway. 
Treated wood in my experience has a very high moisture content, left  over from 
the treatment process. Unless it's been drying in your garage attic  for a 
couple of years, I wouldn't use it.
 
Rummy