Rare Dodge Dart GT? Which engine is this?

Welcome. Keep your enthusiasm regardless. Once you see the side of the block to get the casting you will have a better idea of what you have. The fact that it has been mentioned the intake appears to be an aluminum from the past is a sign somebody was after performance. Unlikely a 2bbl on an aluminum, but it could be. The stock exhaust manifolds mean nothing. You can't get a reasonable set of headers today, let alone back then. It has a chrome box ignition, I'm pretty sure that is not factory. The tires in the old pictures don't look to be the same as the newer pictures. Really hard to tell but they look small, like a cheap set of lower gears.
What is on the roof? Does this thing have a sunroof? I'm not familiar with this body at all, but they are cool looking little cars.
Good luck on your adventure. FABO usually has the best advice and knowledge around.