Thanks Destroyer, bought one in 1985 when dealing with the all the phillips head bolts on Jap cars/trucks. How did they get them so tight without messing up the head anyway? Freeze them first? Definitely one of my favorite tools but don't fit where I need it most. IF I need to dismantle the clamp, that is a winner, followed by the blue wrench, a set of easy outs, drilling, swearing and finally heli-coils. Spraying it all with Blaster every day for a week before I begin. I need to muster some patience before I even start. Considering taking the lower and powerhead loose right off the bat and doing what NYMACk did to his 140, blast and repaint all my corroded parts and pieces to make them look better. But then I want to use the boat ASAP. I am torn.
You probably know about it already, but for those that don't, you can buy a great tool at Sears called a hand impact driver. Basically it holds a socket, or a screwdriver bit, and when you hit the head with a hammer if provides a turning force in the direction you want to go and at the same time the force of the impact keeps the bit seated deeply in the bolt so it doesn't round it out. I've found it invaluable to use on phillips head and allen head bolts on motorcycles. It's great for breaking corroded bolts free and it saves a ton of cussing and time. You can buy cheaper units at Harbor Freight and other places, but the Sears one seems to be made exceptionally well. I have several, and the Sears unit I have is well over 30 years old and still works like the day it was bought. You can buy adaptors to hold 1/2" drive allen wrench bits. 
To avoid this in the future, don't forget to coat the threads with never-seize when you put it back together.