I'd be more inclined to take the cam off than the exhaust manifold.
Managed to spend a bit of time on this at the weekend and come to the conclusion that the head gasket between cylinder 5 and 6 has failed is this a known problem??
Now waiting for owner to decide weather they want to procede and have the cyl head taken off.
Might be a slight warp at that end of the head,in need of a skim maybe.
Did you notice if any of the head bolts at that end felt a little slacker when dismantling?
For anyone who cares P38 all up and running now,
head gasket had failed between cylinders 5 and 6, and because owner carried on driving it till it stopped it damaged the head,
also water pipes corroded through so a replacment head fitted, engine oil was like syrup anorl