I did the spray foam trick with My Chevelle about six years ago. I can't say I felt any change in structural rigidity however it make a noticable difference in sound deadening with minimal weight gain. I put in dead areas in between panels, like below the C-pillar, inside the brace below the B-pillar area and some of the kick panel area. I noticed a decrease in "drone" noise when on the highway.
One thing I don't know if it has been mentioned already but I believe it to be fairly importaint in any vehicle with a cage is to improve how the seats mount. Found small bolts through sheet metal wont be enough to hold your strapped in but in place when the rest of the car is no longer flexing or crumpling. If your seat tears loose a big headache will follow.
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Dan 
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