1978 F150 4X4 351M ready to add insulation inside the cab on the firewall. Old insulation was torn and rotten due to some water leaks so it got thrown out. Should I get the factory style insulation to go back or can I use the Dynamat I have on hand to replace it? The factory style is a bit expensive even on sale its $170 plus shipping.

 

The factory style does work, but I still want to try other things first. The new designs of the original style are good, but agree that they're expensive. And hard to work with if you need other holes and such.
AT least the ones I know of are, with their molded plastic covering over the similar-to-jute backing.

I'm a fan of things like Lizzard Skin spray on sound deadener and spray-on ceramic heat barrier coating. But they're expensive too, and you'd probably have enough to do the entire cab (which would be a great thing by the way!) which might be more than you're planning right now.
So yeah, some Dynamat for sound (and a little bit of heat) and then some Quality Heat Shield with the double aluminum foil backing would work nicely, be less expensive (at least by a little) and easier to work with.

I guess the Dynamat Xtreme (is it?) with the aluminum foil topping is better for noise and heat both, than just Dynamat regular. I've used it on the outside of my firewall in addition to the factory installed interior stuff, but don't know how well it's going to work yet.

Good luck. Hopefully others have tried new and exciting things with good results.

Paul