78 suggests B2/B3. Does the generator cover have 3 screws visible from the outside like your B1?
It is very possible the engine is not the original fitted one but if you want to give me both engine and frame No’s I can tell you for definite.
The 16″ drum didn’t appear until 1982 with the CSR (M) and the Y1. They are distinctively different in that the CSR has chrome rims and the Y1 has alloy rims as well as the number of spokes being different. However again these could have been changed at some point and might even not be from a 750 twin.
I know nothing about springers apart from the fact they ain’t cheap so sadly cannot help you there.
I also have quite a vast amount of spares and may be able to help you out with some bits. I will send you a PM
