nobull1
You can go to a local radiator shop or parts store and they should have books with outside demensions and maybe match to the one you have. I made a home made heater for a Mustang using core out of a ford pick-up and a blower fan from an old flat bed. Plumbed it into existing factory inlet and outlet. I had it mounted to the center hump between the foot pedals It picked up air from the floor and directed to bottom door, very toasty. Outside dem was no bigger than eight or nine inches on any side. This set up would be too big to use anywhere else in the cab.When summer came it was simple to take out and store until winter.
Hope it's just a loose fitting clamp