I considered doing the same, and looked at all the mentioned conversions, and then some (David Harms, Noah Schwartz, Patrick Putze). I ended converting to Polaroid 600SE
type back for 3x4 pack film and 120 rollfilm interchangeable backs. Mainly because the better 4x5 solutions required more advanced tools than I had access to. In better I mean solutions enabling interchangeability of different type backs, roll film, sheet film, polaroid etc, and at the same time still being flush with the camera, and not sticking out under it, or obstructing the viewfinder etc. Only three solutions I've seen fill these criteria, Byron (very complex), Schwartz' low profile back (need CNC milling machine) and David Harms' solution (a very intricate solution of shaping of a host of aluminium profiles and bolting them together).
Should I reconsider and build one at a later date, then I would limit it to double darkslides and machining one of the holders to function as a ground glass insert. Those that have done conversions along these lines often have very simple yet attractive and functional solutions.
This one for instance: http://www.flickr.com/photos/mikeoss...in/photostream
or this one: http://www.flickr.com/photos/zokyo/5113902696/
And looking back in the RFF archives will yield these conversions: