Have you called JBA or Mustangs Plus? I would expect them to be in the know on this subject.
To make adequate clearance on the 1st Gen big block camaros, GM offset the engine to eliminate this problem with special mounts.
Some folks dimple a tube to make them work, others will reshape and rebuild headers completely to gain clearance. There are several adjustable engine mounts out there you could try but. . . if it moves the engine, it is also going to move the transmission which can require shifter relocation, crossmember modification and driveshaft length changes. anytime you change driveline angle, you have to correct pinion angle. all in all it can open a can of worms.
I would call them first.
Then I would lift my engine up and install the header and see how room I actually needed. and if denting the tube would be a viable option.