Have an overheating problem

[biggercb]

I have a mid 70's cobalt inboard/outboard. It idles nice and cool, then when you get out on the water it gets real hot real fast and stays that way. I cracked the drains on the exhaust manifolds to determine whether or not I had any water flow. The starboard side was pissing fine while the port was pissing steam. At first I thought blown head gasket, but all the plugs were fine. I think my maninifold or my riser is cracked and/or leaking.
Is it safe to ditch the engine cover, slap on a set of automotive headers up-side down, and see if my engine temps will stay down???
Is this possible? If so, how do I reroute the cooling?
This is a very budget project and I would like to be sure this is my problem before buying expensive manifolds.

[rrrr]

When was the last time you changed the water pump impeller?

[GofastRacer]

Sounds like a blockage problem, before you start changing stuff out trace the problem, undo the lines and blow air thru them and if you have a pump with a rubber impeller that's more than likely the problem, take the pump apart and check it if you have vanes missing then they are pluging up a passage somewhere, follow the path until you find them, that will solve it!..

[biggercb]

Pulled the pump, the vanes are all intact but distorted. They are permanetly curved. The pump housing is blistered on the outside from heat.
I am going to go ahead and pull the heads off and check for a head gasket leak. Wont hurt to have them freshened up anyway.
A retired boat mechanic told me that I can have the manifolds pressure tested for leaks.
Inspected all of the hoses, no blockages of any kind.
Hopefully by the time I replace the impellor, seal the heads back up and pressure test the manifolds, I will have solved the problem.
Thanks for the responses.