Hi,
I found a patch of mineral wool between the heater core and the case on three sides of the core when I reworked my fresh air heater. It was worn and I left most of it out since the gap between core and case was quite small (about 1/8" or 3mm). I didn't have any ill effects so far.
Helmut

Pictures of my '54 3100:
http://s744.photobucket.com/user/DonkenAndToivolaRR/library/Stovebolt