A small mod is necessary if you fit a LATE HEAD on an EARLY BLOCK, but the reverse [early head - late block] should be a straight bolt up deal.

The A14's left side engine mount sits some 50mm [or so] further back than the 1200's, but I think that the manifold will still fit up OK. From memory, the 120Y/B210 GX used the same exhaust manifold but on the later type block & that worked fine.
Are you sure that powder coating the exhaust is OK? Do they make a suitable powder coat for exhaust manifolds?

To make it all fit & work like a factory deal, you will need the 1974 & later crossmember so that the engine just bolts in & to help pull off the deception, you could make an alternator mounting plate to allow your electrics to remain on the original side.

The distributor advance curve is more tailored to the cam timing, so a mild 'sports' cam that is close to the original 1200 GX timing will be just perfect.