If you decide that 600mm or 750mm might work for you, check over on APUG. Kerry Thalmann has several Docter Optic focal lengths for sale. A couple of them are bound to cover 12x20.
If you have a 450 Fuji C f/12.5, it just barely hits the corners of 12x20 at f/45.
I can't imagine a 1000mm lens on that format. 24inch Artar was all my old Folmer and Schwing could handle without "schwinging" in the wind. As it were.
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