Exhaust manifold crack on ISC350 and effect on power.

I also have an exhaust leak, but it is on an ISL 400 hp. I don't see any cracks in mine unless it is on the underneath side. I do notice that there is some gasket material missing at #4 cylinder and black soot around it. I am having the gasket replaced next week and possibly the manifold if it turns out be cracked underneath. It is a 2 piece manifold and I have been quoted about $550 for it. I hope it needs just a gasket. I have been putting PB Blaster on the bolts to hopefully cut down on labor time.