That does seem to be a common problem I have seen several over here with that. As you know the Y-piece is different & longer on armoured Humbers & don't think I have ever seen occur on those. Although on the Hornet I did have the front manifold crack.
Ferrets seem to get round the problem by having a gap & once heat expansion takes place a sort of 'seal' is formed.
Rather than shims I use copper covered asbestos gaskets. I found that gaskets for a Vanguard car will do. Although the 4 holes are not quite centred because the holes are larger they fit the studs fine. As the 'square' gaskets are not flat-sided but elliptical I have trim a small section where they are next to each other.
I note there were two types of Y-piece fitted to the GS
FV175019 from chassis 20,000 to 20,681
FV228288 from chassis 20,682 onwards
So I don't whether that was to address the cracking problem or indeed how they differ.
Interesting the official recognition that the 'Commers' are proper Humbers after all
