I have a PB-4 bellows. I want to mount a large format lens on the bellows so it can focus to infinity and I will have a very versatile tilt/shift bellows. So I am just wondering if there are any suggestions - I was thinking Mamiya RZ67 because of lens availability, usability, and the flange focal distance would allow me to adapt the lens to mount on the bellows AND have enough room to focus to infinity (44mm canon flange focal distance + F-mount adapter = 46.5mm + 30mm bellows = 76.5mm. The RZ67 is 108mm - I would have plenty of room.) I just need to modify the mounts in a way which DOES NOT extend the flange focal distance in the form of a dumb tube. I just need to buy the mounts really. Where can I just buy a mount? I have a friend that is an expert welder so if it were aluminum or stainless, it could be welded. I just really want to have a bellows that can focus to infinity. thanks.
The RZ Mamiyas are all part of that Medium format system. It sounds like you are re-inventing a bellows type (large format) camera. If you compare resolving power and other properties of LF to MF lenses, you might find it's economical to get an LF (bellows) camera and lens that will fill your needs. A Large Format camera will be larger than a PB-4, but will have all the movements, usually. There are some LF cameras in the crossover range, such as a 2x3". There many 'field-cameras' in these formats for sheets and often with roll-film backs (MF) such as Graflex and Linhof.
If you go to attach, say a digital or 35mm film camera to a LF camera, you can get an F-mount or appropriate Canon mount from an extension tube or a lens reversing mount.
Or sounds like you don't have the lens yet, you could get an LF lens and adapt a lensboard to fit the PB-4 - should be pretty easy, but you'd have to find one, like a 'press lens' that is small, so it will fit the front of that macro-bellows and allow for some movement.
To get the parts to adapt an RZ lens to the PB-4, probably best to adapt a lensmount from a broken camera. I'd check with camera repair and conversion (IR + UV) shops and check used departments at the biggies, like B+H, maybe Roberts, etc.