That would be the best course of action, but I think that this frame is much rarer that most other specials built around the CB750 as there seems to be one that DHolda built and one that was copied by the Honda-Legends group, both of which are in Europe.
I would also agree that the easier way would be to have one built but the cost is prohibitive as I would think that this frame made by a shop would be $1500-2000. I may not get miles under that number but I can also spend the money over time on things like building tools/jigs, as well as gathering the raw materials like tubing, steering neck and other pieces to complete the frame.
The real end goal is to build something for myself from the ground up and with as few parts farmed out as possible. Again, as with my other build, there will be a long list of unique parts from aftermarket sources as well as my brain that will come together to make a very unique machine.
The largest hurdle besides the frame will be to get acceptable bodywork (Seeley seat and modified tank without backbone tunnel). My first choice is aluminum but it presents a decent price barrier for me but also isn't made by my hands. I would like to experiment with making a buck from wood/foam that I can vacuum form fiberglass over and make my own. Of course fiberglass comes with its own set of issues when ethanol gas comes into the picture.