What heat insulation product to use for Duster interior?

That is the stuff, but they used to have the double sided version. I too have a water leak in my '68 from under the dash, and yes, fix that first. I ordered the wiper gasket kit from Schumacher, but it is available elsewhere too. I am also going to replace my front windshield gasket and windshield, as it is likely leaking too. Taking car of any leaks is definitely the first step of my interior restoration. I have a convertible too, so I replaced the top and all those seals as well.

I found significant water in my insulating mat and carpet after my first attempt, which worked great, but had to be removed because of the water intrusion. There's no rust in my convertible and I don't want it to get a toehold in there.

For those who would like to deaden resonance and block some sound coming through their sheet metal, check out bulk packs of Dynamat Extreme from Amazon. Sign up for their shipping program and get free shipping. It comes out to about half the retail price when I last checked. Their are other products that work great too, just shop around. Avoid the Lowes roofing stuff though. It does get smelly and loose when it's hot out.

For fixing leaks and seam issues on the floor pan etc, I like the 50 year, paintable, black roof caulking in the tubes. The kind that fit in a caulking gun. It dries fast, looks good and can be painted.

After I finished sealing any pin holes etc, I painted my floor with that paint on bed liner available at Oriely's. The stuff is oil based paint and it sticks and covers in a nice hard finish. It's scuff resistant and durable as you would imagine. I really like it for a sealer or even under carriage paint.