Bruce, fun job eh? I believe that is only the second upper rubber foam gasket I've even seen - and I've restored a good handful of them. Guard it with your life! Okay maybe not that valuable. Do yourself a favor and make sure the gasket was centered on the opening when installed. I installed the client's heater last week (fun, fun job) and while under the dash, looking at where the studs were in relation to the U-shaped mounts, it appeared as if the heater had to move about 3/8" more towards the right. However it was right up against the water hose to inlet manifold (Note: Jay Mackro, your heater hose is about a foot too short to reach the fitting on the inlet manifold). Obviously I couldn't move it any more but with 5 mm fender washers it would hold the U-shaped metal pieces on the heater. Now to me this is weird: Since I have the front air inlet cowling off (and standing outside) I stuck my hand in and felt the edge of the foam on either side of the exterior air inlet. It indicated the heater was way too far to the right.
Not sure if you are following this but on the right side about half of the 5 mm fender washers are holding the heater on. On the left it is just the opposite and they are nearer to the round part of the U.
If you're with me, Bruce, but hopefully you've not painted or removed the paint from the side panels on the heater. If not, look very carefully at where the old washers were centered on the U-shaped mounting brackets, make a mental note, and put it back exactly where it came off. I'm sure if you're off some, not a big deal, but always best to reinstall it as close as possible as to where it was before.
I know you've visited my heater page on my website:
Biba Restorations Round-Tail Spider Heater Rebuild but here is another photo of the .25" dense white foam I use in place of the yellow stuff.
Biba