I did actually look into the original 180 degree V12 engine. I found some drawings on-line and in some of the books on the original car. The engine is impressive, and would be an absolute treat to have. I did quickly look into the engines fashioned from multiple blocks - and trust me their are some weird ones out there. The twin flat-6 is long, and I have to believe that its not very reliable. Some enterprising souls had built some V-12's from twin 6 inlines, so I'm sure that a flat-12 could also be built, but I'm not aware of any air-cooled inline 6's. Again, it probably only just works.
But I need something reliable and not insanely expensive, so I will be basing my drivetrain on a twin-turbo 993 engine (just the 6 cylinders). My plan is to try to layout the components like Mark Donohue's 917/30, and unlike the first year of the turbo 917, I will be using blow-off valves
I find it amazing that they raced some races with turbos and no blow-off valves - the drivers reported having to take their foot off the gas a full two seconds before they wanted to slow down. Those guys had balls of steel (or in Porsche's case, it's probably cross-drilled titanium with a ceramic coating passing oil through them to save on the weight of hoses).
The one change I will make to the 993 is to have a horizontally mounted cooling fan like the original 912 engine. I'd appreciate any thoughts on how to do this. I have seen some photos of more contemporary flat-6 Porsche racing motors with a horizontal fan - anyone know how this is driven, and what shrouds might be available?