looked at your pics and i'm afraid you will always have that problem : i see the plexiglas so close to the headers running above your engine .... don't know if they are coated but still... perhaps wrapping them with that heat shield strip-material would help ? Think if the car runs wel some more/less fuel won't affect the headers temp so much ? They come hot , very hot anyway
Remember the GT40's used aluminium louvers and still had those deformations by heat...
Isn't a change back to original better ? all that heat in driving wind under the car instead of in the trunk, which space you even lose by that configuration ??? Is it worth the few extra ponies and racy look ??? The HP gain will be in the cros-over pipes i think, so a optimal sequence can be achieved in pressure/vacuum on firing cilinders no matter if it comes from the right or left bank. Stock headers are ofcourse completely divided right/left ... but i've seen a pic from one who had a balance pipe under the car , just before the mufflers. Don't know what gain is there to see ... ?