I too have seen a large number of nine bob note manifolds

I've generally had pretty good luck getting the turbo to manifold studs out but I always start with lots of heat from a MAPP gas torch
I also removed all of the manifold to head studs using the same method

My routine goes
Soak in WD or a diesel/brake fluid/light oil concoction overnight
Heat the bejeesus out of it with torch
spray again with WD for a shock cool
heat again
undo

One tip from Terry_T is to give the manifold to head studs a good clout before starting as this gives the corrosion a good shock and helps to break it. Not so easy with the head in situ but with an extension bar you should be able to do this to a good few of them

The manifold I am using in my build was also warped and required skimming. It was quite obvious with any flat edge.

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1996 ST205 UK