i am no expert but i do own a 72 roadrunner with a air grabber hood.
they put 400 engine decals on the fenders above the side markers on 72 roadrunners with air grabber hood. my car had one when i bought it 20+ years ago.
I am not sure about a 440 in the same spot on the gtx.

I think there should be a gtx emblem on the 1/4 panel above the lower body line by the door.
i have seen road runner emblems in this spot also on gtx's. i don't know if it's correct.
and i don't think there were any engine decals or emblems on the 1/4's at all.