I can't offer any advice here I'm afraid, but I would also like to know if the Holbay exhaust will fit.
In the obligatory "boxes of bits" that came with my recently acquired Series V there is a recon Holbay block with Webers, inlet manifold, and 4 branch exhaust (which I assume came from the Holbay Hunter). The inlet manifold has been cut and welded to "straighten" it out for the vertical Alpine engine. The exhaust has also been cut and welded in a similar fashion, so the previous owner was certainly under the impression it would fit.
Are you measuring up a modified exhaust?