Jim Carter offers the blower motor rubber seal & the ring retainer.
http://www.oldchevytrucks.com/cart/product.asp?prodid=MEH200&i=|||||
http://www.oldchevytrucks.com/cart/product.asp?prodid=MEH254&i=|||||
I have run into similar problems when I restore Fresh Air heaters as most get updated with 12 volt motors.
All of the 12 volt motors are smaller in size compared to the original 6 volt motors.
The 12 volt motors are too small to fit the hole in the firewall or the original seal.
I have devised an ingenious

work around for getting everything to seal { & it will work in this case also }
First get an old original 6 volt blower motor that is fried.
Take it apart & get just the front cover off the motor.
Slide the cover into the firewall seal & retainer.... & silicone it in place.
Then take the cover with the seal attached & install it over the new {smaller} blower motor protruding out from the firewall & secure it with 3 sheet metal screws.
The finished installation looks just like an original blower motor installed on the firewall.