nice project! We're working on converting an old Graflex 1A in a similar manner. Not finished either, but getting there slowly.
Have you considered using a 5" Ross Xpres? They can be great lenses and are quite shallow as well. The normal 5" measures a little over 3 cm and the 5" wide-angle (not from the Air Ministry but a civilian one) measures 4.1 cm, so maybe one of these should fit. The Air Ministry Ross lens can be gotten cheap, is slightly more shallow, but it has limited f-stops.
Both our 5" normal Ross and 5" wide angle should easily cover the 612 format without vignetting. They're relatively fast lenses as well: 4.5 (and sometimes available even faster, if I remember well, but way more expensive).
What did we pay for each? Well, the 5" Ross came with the Graflex for £70 and the 5" wide angle Xpres + epsilon shutter (both in good condition) set us back £35 (excluding S+H). For us that falls in the category 'reasonably cheap'.
Personally, we prefer a Ross Xpres over an older Angulon any time for it's special character, somewhat Heliar-like in plasticity. (Agree with that, mr. David -my middle name is Heliar - Goldfarb?)
Hope this helps,