For any smaller format my advice would be: "build your camera first, and then look for the lens".
In your case, you are after something that's VERY difficult to find, so patiently waiting for your chance is the only way to go.
From the context (home built camera) i get that it's not exactly the case where money is no object
I am afraid that any lens which meets your criteria is either expensive or VERY expensive.
A decent narrow field copy lens in that focal would be expensive. A slightly shorter Dagor (but with far more coverage) would cost even more. Probably a 480mm Plasmat, like a Symmar, would be somewhat cheaper, and on top of that would come in a modern shutter!
I am not positively sure about coverage, you should check Schneider literature. Ptobably the answer is yes, cause the various generations of Symmars cover between 70 and 74 degrees (a 300mm covers 10x12").
Narrow field graphic art lenses have restricted coverage (most under 40 degrees), hence the need for much longer FLs.
Wide field graphic lenses don't go past 355mm (like the G-Claron), and ultra wide angle graphic lenses are either bad for general use (G-Claron WA) or unavailable in longish focals (Apo-Graphigon).

My suggestion is:
check the max coverage of the G-Claron 355mm, and limit yourself to that format.
If you want to save, buy a first generation one in barrel, and then look for a Xenar 210mm on Compound No.3.
The shutter will be a perfect fit for the old G-Claron!

The only other choice would be some oddball graphic lens, like a JML, Ednalite, or something like that, but i've never spotted one in very long focal. Even Copying Ektars, which are not exactly cheap, don't come in such long focals, for what i have seen.
Anyway, the Bay of E is the only likely source.
Trying your luck here, or on other forums, won't do anything bad, but the chances of success are slim.

have fun


Sent from my Android tablet