Coming in late Slunnie, but I can confirm that series 3 (4 cylinder) and stage 1 bulkheads are identical. The differences are in the bolt in panels in the tunnel area.
Chris
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I presume you could Slunnie.
I only know they are the same as I have transplanted a 4 cylinder bulkhead in to a v8 stage 1 I once owned, to replace it's own rusty bulkhead. There is a bolt on panel at the bottom of the bulkhead that you change to convert from a series squared tunnel to the curved tunnel in a stage 1.
Cheers,
Chris