Thanks for the inout guys.

Doing some further reading and trying to decifer all the information and have come up this. Let me know if I'm on the right track.

If using S13 front brakes and R31 rears (or similar), I will need a 13/16 or 7/8 size master cylinder. I'm assuming 7/8 will be better as per unfamilia has stated?

A booster is not required but optional. I had a booster on my old coupe and liked the way it stopped so I may opt for it again.

Suitable booster swap is a Lancer (Direct Fit) however needs mods to work with a Datsun MC. Can also use a 120Y Booster with mods to firewall? If using either of these boosters, what is a suitable 7/8 or 13/16 MC? The 120y dual system is still 11/16 isn't it?