In any correspondence or verbal contact with DVLA always use the word 'refurbished' , not rebuilt or reconstructed, otherwise they may point you to a Q plate.
The key components re the age are Chassis (Frame), Engine, Wheels and Brakes and Forks. Provided yours are from that period, which it sounds like they are all original, then you wont have a problem, as the BSAOC dating certificate will reflect that its the original engine, or if not it will say how old the engine is. They go by the youngest components so re the other parts, if you fitted say 1983 Meriden/Harris Bonneville forks and disc brake then they would be looking at a 1983 age related plate.
If you want to fit younger parts, use the originals to get the plate and what you do afterwards is entirely up to you.
Hope this helps. (If any of this is incorrect, Julian will point this out).
Chris