Re: Rivet spacing
I agree about not cutting to old window frames as I didn't plan on doing that either. Pop riveting Angle iron might be easier to shore up the top and bottoms and use an automotive grade type of sealant or the Great Stuff where the gaps seem to large for the automotive stuff. I used the great stuff on the top of my step plate rails which i placed the metal on the great stuff before it cured and riveted it down. It deaden the sound Immensely, the contractors use it around here as they say it cuts down on squeaks plus they put it under the bath and shower also, its makes a difference in the feel.