Hello Brian,
there are no differences in the port or stud spacing. So a 1275 manifold will fit a 1098.
George, if you have an after market manifold, and have problems doing the swap then there is something wrong with the manifold.
My wife has a Morris 1000 Traveller which had a 1098 engine and I fitted a three branch tubular manifold and an after market 1 1\2" S.U. manifold. I subsequently fitted a 1275 head which the manifolds fitted with no additional work.
Alec