This is what I ended up finding and using on my car. The difference between open and closed cell is the density of foam or squishability. Open cell will squish easier, but use what ya can get. Closed cell probably won’t deteriorate as quickly, but is a little harder to use. You won’t use much of any size, so buy the shortest roll of each you can. Below is about $20.00 worth of foam (IIRC). Some of below was a Poly foam, some vinyl foam. Use Poly as your first choice, as our heaters put out a lot of heat, and not sure how well the vinyl will like that, especially closer to the core. IIRC, I ended up going to a few different stores to find these sizes. Don’t neglect changing all the rubber hoses on the core, and at heater valve. Don’t forget to cover up the holes at the top of bellhousing before starting disassembly. See what ya got yourself into?……it ain’t that bad really. When you dismantle the heater box, you will see just how much the original foam has deteriorated over this much time.
3/8” T X ½” W open cell
¼” T X 3/8” W closed cell
3/16” T X ¾” W closed cell
1/8” T X ¼” W closed cell ---- used on the heater core itself to seal perimeter, and prevent rattling in the box, do use it.
½” T X ¾” W open cell
3/16” T X 1- ¼” W closed cell