Interesting engine, that - but it was a 4-pot, not a 6-cylinder, which made it all the more unusual. Very torquey, like all good boxer engines, it suffered from a lack of development (high camshaft wear and the infamous power steering problem, where full lock on a cold morning would put too much load on one camshaft and cause the timing belts to skip and drop all the valves on that bank
I did consider the Gamma engine and transaxle when scheming up the Nova, but there are so few left that sourcing spares would have been a problem, and the torque characteristics would not have suited a smaller, lighter car (need more top end!).
Other than the ubiquitous Porker, one other flat-6 that might have worked well would have been a Honda Goldwing 1500 unit, but we are back to ricers again
Porsche developed their whole line of race engines from the 4-cylinder boxer unit - first a flat 6, then two flat fours made a flat 8 (908), add a couple of cylinders to make a flat 10 (910) and then put two flat 6's together to make a flat 12 (917). I'm sure Alfa could have expanded the Sud boxer engine to a flat 6 for a FWD "executive" saloon had they had a mind to. Carlo Chiti oversaw the design of a flat 12 for the Tipo 33 programme. The thing is, making one up from scratch wouldn't be easy...
There's always the excellent Alfa V6 of course, but using that would seriously compromise my roll axis, and unless I mount transversely (which moves the CofG backwards as well) I'm stuck for Italian transaxles - there's only Porsche (mounted upside down), Citroen/Maserati (if you can find one), Lancia Gamma (ditto), Audi, or mega-expensive ZF and Hewland.