How to get the manifolds off?

["BIDGOOD, ALAN" <BIDGOODA@pab22a.ssd.loral.com>]

Spring is here and I've started on a carb rebuild
and replacement of the exhaust/inlet manifold gasket.

well I've undone all the nuts holding the manifold to
the head, and it just won't come loose.

I've tried to lever it, bash it with a hammer (not too hard)
but it just won't move.

I wouldn't bother, but I have been getting exhaust fumes
under the hood and it won't idle smoothly.

Any ideas how to get the manifolds off?

(My car is a 1978 MGB Roadster)

Thanks,
                     Alan B.'