I was replacing the motor to change voltage so I didn't care about the old motor. Found the same problem you did. I drilled/ground the heads off the bolts that hold the motor together on the exposed end of the motor. Took the motor apart and allowed me to support the back side of the fan on the outside. I was then able to tap the shaft through the fan and out. It worked for me...
Just curious, had you considered keeping the original heater motor and installing a voltage reducer for $10.50.
http://www.oldchevytrucks.com/cart/product.asp?prodid=EL135&i=|voltage%20reducer||
