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Anyone ever done it? I'm building a hot 850 and afraid that the Hs2 carb will be a restriction. However, if fitting a H4 on that manifold maybe the manifold will be the restriction.
I'll be opening the hole of course.

Not done it myself but I did see an H4 with a pancake filter on a cast iron HS2 manifold for sale somewhere recently. Maybe it is possible to fit under the bonnet.
I've gone for the twin HS2 route.

What is the plan for the hot 850 motor?

It used to be quite a common mod in the 60s but became obsolete with the increasing availability of the HS4 and manifold.
You can just take off 1/8" all round the hole as the stud pattern is the same.
Clive Trickey (Tuning The Mini) claimed moving both studs up by 1/4" and taking more off the top half of the hole would give better flow. Probably not worth all the hassle now though.
If using the H4 you will need the 30°float chambers. 4814 or 4815. Though you can get by with 20° ones.
I agree that any gain from the H4 with its longer choke tube will be offset by the relative restriction of the manifold, particularly if you are hoping to hot up the 850. The bends are pretty angular. The later hs4 inlet was a better shape.
Why do you want to stick with the basic cast iron manifold? Would you not be better using a separate inlet manifold, with say a cooper three branch exhaust.?
I have a similar set up on a 998 using a freeflow exhaust and an old inlet matched to the H4. I'm not sure if one could be picked up fairly easily now.