there are 2 speednuts from the inside. they are either 3/8 or 7/16th. you will see a couple of round holes in the firewall cover and there they are . you will require deep sockets. 2 studs come through from the heater box to the inside
ron

http://s908.beta.photobucket.com/user/padresag/library/turned%20works
http://s908.beta.photobucket.com/user/padresag/library/furniture
and a little history of where I come from
http://forum.woodenboat.com/showthread.php?156542-old-boatshop-in-Steveston