I am trying to take the lower unit off of my 1957 25 HP Sea King, I have gotten all the screws out but one which was a lot of work. The final screw will not budge. I cannot get leverage using a screwdriver the typical way so for all the rest I had to use the screwdriver horizontally and they all worked due to more leverage. But this one will not budge and I am afraid I am scratching to much off the screw head off soon if I continiue using the same horizontal screwdriver method. It just wants to slip off. Right now I have it sitting with WD40 sprayed on it. Waiting for some insight... Any special tool to help?