Look at it this way... if you had an '84 Chevy El Camino, and installed a Buick Grand Nat'l nose & engine, would it still be a Chevy, or a Buick?
Well, in my eyes, it would still legally be a Chevy, due to the vin #, but I would still tell people it was a Buick.
Your engine in question... the block numbers would still indicate it as a flathead, so flattie it is. It it still a flathead, but modded with Ardun heads. IMHO, or course.