[mart_it]

Those Manifold's look clogged completely!!
I'm surprised your motor didn't overheat from not being cooled efficiently.
Are you running a thermostat?

It seems the right side bolt can maybe come off with a socket. I would use a punch and hammer the head of the bolts before trying to put a socket first, see what it does....if not cut the heads like you said.

If the manifolds aren't cracked why replace them? Unless your running in salt or really brackish water, I would get them cleaned and blasted, then re-painted.