I doubt you're short-changing yourself on maximum air-flow with that housing's two intakes veale. Unless your 351 is a deep-breathing hi-po engine and you like to wring it out now and then.
What have you got in the engine by the way? Do you like to rev it up? Or do you drive it like the truck that it is?
If you want to try more filter, you can always just pick up a taller air filter element to fit into that same housing. That way you have a low-hassle way of finding out if more area will net you anything, without the bigger expense and hassle of changing the whole thing out.
You can spec out a taller K&N or other performance brand, or even a paper-type element perhaps(?), to fit into the same diameter (14" maybe?) as you have now, but the taller element (maybe between 3" and 4" inches?) will allow more air flow still. Yes, you lose your cold-air intake properties (as the lid will now be lifted up and taking in under-hood air again), but it's an easy way to get the larger filter area you were thinking about. And it eliminates the snorkels as any type of potential bottle neck. If in fact they are.
Ford designed that setup to support the 225 (net) horses that the Mustang was probably developing at that time though, so it's likely doing a good job for you, if your 351 is a low-revving setup. If it's all hi-po stuff though, you might benefit more from the larger filter after all.
And though I don't think you'll be able to tell much by the seat of your pants just by plumbing the snorkels for cooler air, I still think it's the right thing to do. Hey, they're there after all. Might as well use them the way they were meant to be.
It's just a "better" way to do it.
The taller hood can let you play with element heights. And if you figure out a way to seal off the opening when using the taller element, you'll still have the best of both worlds.
That's what I'm doing on the 400 in my '79.
Paul