Bennie

Early 1949 mount are not the same as late 1949 mounts

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0102.htm
Look at fig 1 and fig 2 to determine the correct mount for you

You should not have to modify if you have the correct mount

If you have a late 1949 the mount will look like this
rear cab mount pic

good luck