Sounds like you have everything separated so why not drill, tap and install the relevant new manifold bolt pattern on the oval port head (if it needs it) or just bolt the round port manifold to the oval port head if it shares that bolt pattern already.

Don't think you need an A12 carb at all. Bike carbs would be much better.

Can you upload a picture of the Janspeed extractors, is there much of that stuff around (or do they still make gear for the A-series?).