The heater motor sticks through the firewall...I'm thinking its a fresh air heater. There is a metal trim ring that has a gasket behind it that goes to the outside of the firewall.(under the hood) Mine was already out so I can't give you "exact" details but I think the bolts go through and hold the motor...but not sure. You can make your own gasket out of just about anything.
If you don't find it send me a PM and I'll trace off mine and then you can make your own out of a piece of some appropiate metal. I have mind laying on a table now.
If you don't have a repair manual you will benefit from getting one. There are some on line if you like to check them out.
Since I haven't actually done my heater yet someone else that has will come along soon and maybe help you out more. In the meatime check out the link below.
http://chevy.oldcarmanualproject.com/
