i have heard of conversions where the camera had what looked like a funnel
attached to the 4x5 back to choke it down to 3x4 ..
and i was under the impression from what people who had this sort of thing done
( well sort of half done seeing the fellow who was doing the conversion had a massive stroke
while the camera was apart on the bench )
the back needed to be shimmed to keep the film to mirror distance and ground glass to mirror distance
the same ...
maybe they were pulling my leg, i am pretty gullible ...