if its a round port su manifold then will fit round piort head, oval port manifold will fit oval port head... easy to make a bodge linkage from threaded rod, lock nuts and some steel plate for the strikers onto the carbs.

You need to make sure that you ask the right question. To answer the question that you asked, the answer is NO.
The carbs will not fit onto a 1200 engine because the carbs bolt onto a manifold.

If you have a suitable Lynx twin SU manifold, then the carbs will fit to that, but will be spaced too far apart for the stock linkage to fit & work. [I tried it]

If you want to know if a 1200 GX 'induction' will fit a stock 1200 engine, then the answer is no.

If you have a GX head complete with induction & want to know if that will fit a stock 1200 block, then the answer is YES.

Yes I agree you would need the GX head to make it work or get new intake manifold to suit your A12.

If your cylinder head has round intake ports and you are going to use a GX intake manifold, then No.

Will 1200gx carbs fit on a standard a12?
Im a bit cashed up at the moment and want to burn some into the motor. Im not too far off another car so assembling all the parts for a rebuild which will happen wit the new car. Just want to make sure they will fit?
Cheers Ed