Since we are talking here hypothetically...
You would have to transfer the 77 in entirety to this other frame with the Vin tag. The vin number has to match the one the frame, drivers door post (if there), and glovebox. Hopefully, this other frame/vin/tag has been kept registered and current all the years you have owned it. If not you might have to have a Vin verification form completed by the CHP or local PD in order to complete registration.
The former 77 frame, with the vin tag, former CO reg, and a bill of sale to you, would be needed if you ever wanted to sell it.
I can understand why the former owner would want something in writing which states you purchased the vehicle from him. Since the vehicle was never registered in CA, a form from CO would be more appropriate. But if the former owner started any CA paperwork, or if the vehicle had an aborted Smog test, the Vin may have already been entered into the CA DMV system.
FYI.. What you are purposing, on this public forum, would be considered illegal by the State...