Yamaha lower unit removal problem

All my previous boats have had straight inboards, so working on outboard engines is new to me.
Tried to remove the lower unit on my '91 150 yamaha last weekend to rebuild the water pump, but couldn't get some of the mounting bolts out. Of the seven bolts that attach the lower unit,
I was only able to remove two Two others are turning so hard, that I'm afraid that their galled in the aluminum. Two others I can't move at all, and the one in the zinc anode is stripped out. It turns, but won't come out. I don't think I can get penetrating oil into those vertical blind holes. I'm tempted to try to heat these bolts while trying to remove them.
I'm hoping some of you guys may have experienced this problem and found a successfull
way to deal with it. Any suggestions will be appreciated. ???