hi,
i'm not sure about the 261 engines, but
I put my 1950 216 manifolds onto my temporary donor 1953 235 engine to continue using the truck while i rebuild my original 216.
the 216 manifold alignment rings are too sloppy for the larger 235 head ports, and the 235 alignment rings won't fit into the 216 head,
so i left them out, the gaskets seal up everything sufficiently well for me.
i don't see why one couldn't interchange any combination of heads and manifolds as long as the holes and bolts line up.
individual exhaust and intake manifolds for 216 engines with 2-3/4 carb bolt spacing will not interchange with 235 engines 3 inch carb bolt spacing.
so you have to swap the intake and exhaust manifolds as a set, which is easier because separating the two manifolds is usually a pain as all mine are rusted together as a solid unit
