That is interesting...when I saw where the rear control arm mounting area was in the photos in your other post I thought: "Hmmm, That looks like it is really taking up a bunch of space in & back, I guess it's just the closeup of the photos" Guess not.
One thing I will add is while Hooker's Super Comps are one of the nicest set of headers (in terms of quality,power, ect.) they do take up the space due to the larger than "normal" primaries & the fact that they work hard to stay closer to equal length than some others. I helped a friend install a set of those (BIG Super Comps) in a truck with a 502 & we ended up slotting/moving the motormounts a little bit here & there to get ideal fitment between everything.
Maybe you can find a local guy with a set from a Chevelle you could try? I used Cutlass ones made by Headman when I installed the 455 in our truck. Appears to me, they fit the truck better than a Cutlass! Good Luck, Lorne