Must be a lack of imagination on my part. You want to replace a smaller film holder with a larger one. You can't put a intermediate plate in there as that changes the length from the light beam separation path at the divergence point. You can't mount the larger back on the smaller body unless the mounting points are within the 4x5 frame area.
Shimming? Shim what? Move the ground glass on the viewing port? Shimming the back compounds the problem.
Sounds like a hack job to me. Maybe it's just my lack of imagination.
tim in san jose (no, you cannot have my 3x4 rb)