I was reading this page:

http://jaw.iinet.net.au/cars/120y/hitachis.html

and noticed that the setup seemed to be a a12t manifold and carb setup with an a15.
Does this work?
Would the gx manifold ports not be too large for the a15 intake ports?

Yes, the GX or A12T intake manifold ports are larger than A14/A15 size ports. So there will be a port mismatch.

Ok but it would seem to be possible to have a successful setup even with this mismatch.
Does the mismatch matter?

Yes, the mismatch matters. It will be down in HP.

Some guys "port match" the head (grind out the openings to match), which is better but still won't make as much power as a GX/A12T cylinder head.

If you want twin Hitachis on an A15, I suggest buying the factory A14T manifold, which is a perfect fit. There is one for sale on eBay right now, or from GEM.

Ok thanks. Is it necessary to use the a14t carbs wit the a14t manifold, or will other SU style carbs fit, for example, Hitachi HJE38W, SU HS4, etc?

Any will fit, including 4-bolt types.