As you found out, those lower bearing carrier plates are fragile and break easily while trying to remove them. Now I use heat and lots of it on the outside of the exhaust housing to break the bond between the two. I don’t worry about scorching the paint as it is easier to repair than finding a replacement part sometimes.
On the bright side, a bearing plate from any 5.5 or 6 horse motor from ’54 to ’77 will work in your motor but if you want one right now, ME has repops for a cool $145.
http://www.marineengine.com/newparts/part_details.php?pnum=OMC0382796
