Umm, OK although I'm not 100% sure.. again I need to hear back from Bert (at least his advice), but, I've converted the Graflex back on my 4x5 Series D to a Graflok back using an adapter kit that Bert sent me. It's 100% reversible, and it takes less than 1 hour to do. You're taking off the Graflex mount that's attached to a rotating plate and attaching a Graflok mount. Pretty straightforward stuff.
My guess is that it's a pretty similar situation for a 3x4 to 4x5 mod, in that the thicknesses of the 3x4 Graflex mount is probably not that different from that of a 4x5 Graflex mount, and if the rotating plate on the 3x4 is at the same or very similar distance to the film plane as is the rotatin plate on the 4x5, then it's just a matter of fitting the larger 4x5 mount on the smaller plate.
It's not the thickness... it's the size. A 3x4 rb is not the same size as a 4x5. The adapter plates probably won't fit without an adapter panel of some sort. If you have an adapter panel, you have now changed the distance from the film plane to the splitter point inside the camera. Hence the focus mechanism for the 'reflex' will not focus correctly. Kinda like using Mamiya RB67 roll film backs on a 2x3 Graphics. They fit... but the focus is just off.