I replaced the rubber isolators between the cab and cab mounts on my '53. That made a very noticeable difference. Use something like Dynomat everywhere you can hide it - backside of the dash, inside the doors, firewall, floorboard, on the cab behind the gas tank, even stick a piece on the gas tank. I don't recall how the '65 seat mounts in the cab, but if you can isolate the seat from the floor with rubber washers+carpet, etc that will also make a difference. I tried several different carpet pads, sound deadeners jute pads, etc. and found the absolute best thing is a closed cell carpet pad I got from a home carpet supplier. It's a waterproof sound and temperature barrier that works great! I ran it from under my dash all the way back to the gas tank. I then covered that with interior carpet I got from the same place. It's cheaper and heavier than auto carpet. The problem with your truck is you've got lots of flat hard surfaces - dash, doors, windshield, cab roof and even (I'm guessing) a vinyl seat. Not only do these surfaces transmit road, wind, and engine noise into the cab, they also bounce that noise around and aim it at your head. You could modify your truck with a cloth headliner, full door panels, and a cloth seat, or wear earplugs. I'm doing both
