Yes, I get that.
They're expensive drop in replacements.
I still don't get why you 'want to' spend $300 for a 1503, and then gut it; especially wehn the box itself that you're buying isn't Fearful spec.
Plywood and glue are cheap, and GB's Cheap But Good Crossover is not only stouter, but much better designed for the purpose at hand.
Im not saying that mod'ing the 1503 might not get you a better cab, but what are you really saving to get "not the same thing."
If your goal is Fearful, just go that route, and have it right off the bat, and, right the first time.
If it were me-and I'm one of a kind folks!-if I were buying a 1503, I'd use it as is, or, go a different route. Plenty of folks use 1 or 2, and are quite happy with them; and you can buy two, cheeeeeep!
YMMV; feel free to do whatever makes you happy!