Althoug I dont quite understand you numbers on that hood photo. The scoop opening is the small opening on top of the hood, and that opening can not be as wide as 50 cm (or 19,69").
But, as you also mentioned, judging from a snorkel on a stock 70's air clearer, a scoop opening don't have to be very large.
I've also heard (from motorcycle technology) that ram air intakes should be small at the opening and then widen out and be large arund the air filter area, as this will increase the speed on the air coming in.
I will keep you updated!