Hi Geoff
The 250, 400, 440 all used the expansion spring type exhaust manifold part#467, on all years up to and including 1980
The 1981 model 400, 440, 490 were changed to bolt on compression spring manifold part#6893.
Except for the 250's which still uses a stretched expansion spring to hold the chamber on, but from 1981 onwards the arrangement was changed to a spigot manifold that is a part of the alloy cylinder casting.