I think PTFB? showed something about a new shaker scoop for using bigger filters, so might see if they are releasing that soon enough.

Based on the maths of 376 cu in x 7000 rpm(not sure if you rev that high) / 3,456 = 762 CFM rounded up

So for a round filter you'd need D x H x 6 x 3.14 = 762, 14 x 3 x 6 x 3.14 = 791 CFM rounded down

That or keeping diameter stock and increasing height, 12.75 x 3.5 x 6 x 3.14 = 841 CFM rounded up

12.5 x 2.5 is 589 CFM rounded up, so yeah you're definitely under filtered assuming you're revving that high. Even if you're more modest at 6,000 rpm that's still 653 CFM rounded up. Which I'm confused though since looking at the stock replacement for a '78 400 car it shows it being a 12.75(averaging ID and OD) x 3 so stock being 12.75 x 3 x 6 x 3.14 = 721 CFM rounded up, so not sure why you'd have gone smaller than stock.