The slant was to make fitment easier, the OD of the plenum is bigger than the OEM.
You should not need new boots, The clamp should fit just after the chamfer:
The boots won't go as far on the side manifolds as before, but sealing is still good. The grooves are more for locating.
Best way to get it easily on it to first put both boots as far in as possible on the plenum, slide the plenum in between the manifolds, then slide the boots back in their final position.
Don't tighten the outer clamps before you get the new intake tube in, it won't be as flexible as the original one!
Now that I've said that... You're probably right, using this method, the chamfer doesn't need to be that wide! I can probably make it narrower, that will give more surface to seal.
I've put the chamfer when I was using a different method to put the plenum in: Both boots on the sides, slide the plenum in, then slide the boots over the plenum.
Thank you for you're feed back, continue to share your thoughts! This is exactly what I need to know!
Ben