Don't worry Mark you can sell your spare "Dagor" to Don after you fit the Cong
You will definitely get far better results with the Congo.
I now have a 1950's coated Tessar which I can fit on my Ernemann 9x12which has no lens. It's worth getting more 9x12 plate holders as film is readily available, I think I have at least 15 that luckily fit all 5 of my 9x12 cameras.
Let us know how you get on making a 5x4 back.