Re: 396 intakes
You can find a cast iron O-port intake at a bone yard or e-bay for $25-50. What heads (casting number) do you have? There are two sizes of O-port heads,..large and peanut. Let me know and I can give a half dozen intake casting numbers that will work much, much better than what you're suggesting. Or you may find a good used aluminum intake,..what carb are your running? Q-Jet or Holley?
Also,..don't mess with the rectangular stuff,..it was too much for a 396 in the late 60's and it still is.