Here's a 300mm Petzval from Austria-
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My suggestion for a platform is a Speed Graphic 6x9 (sorry, not certain of specific name) with the focal plane shutter. Then you remove the bellows and lens mount and build a box, sliding or not, to hold your long lens (~200mm is max focal length on those 6x9 bodies using the original lens mount). You can probably use the front focus section if you want adjustment, need to swap sliding boxes. This eliminates shutter worries. Give you a stable mount. And easily changed to other focal lengths. Heck, the viewfinder tube is already there in this one-
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Rip off the rangefinder and other geegaws.
You'll need the Graflok back and then get a roll film adapter for 6x6. This leaves open 6x7 and 6x9 down the road.
The big advantage is the shutter is not tied to the lens since large shutters for those focal lengths are both very old and very not cheap.
A while back I was given an old Bush Pressman (?) with a graflok back. Cut away everything but the back mount. Replaced the sliding back retainers with set screws. Built a box up front and used the focus helical from a Pentax 50mm lens to mount a 47mm Angulon. Scale focusing. Well, keep an eye out for Graflok backs to make life easier. Buying the roll film system off the shelf and having an accurate film plane and mounting saves a lot headaches. No sense reinventing the wheeel or trying to duplicate machining old design; just grab and cut and paste.