It seems to be a nice project.
There are always two kinds of re-built,
1-Restoration as per original factory specs
2-Re-build, enhance and modify as per usage, looks etc.
Purists always critisize the modifications in a classic and the others justify the modifications. In the end it is the decision and choice of the owner. I personally think that if you are re-building it as a daily driver then the emphasis should be on brakes, engine - gear ratio matching and engine/drive train noise.
You should get opinion on how 3C engine goes with the original transmission. Is it easy on engine or you high engine RPMs required to get decent speed on city roads. It is an important factor because if not matching, the engine noise will be too much and you will not be easy and comfortable driving in city.
From where you are getting the adaptor made for coupling engine and transmission?