I just measured my Comet, door panel to door panel is 53½"... My buckets(pair) at widest point are 50½", so there is normally approx 1½" clearance between seat and door... From what I remember about the '77-'79 T-Bird & Cougar you can about set a six pack between seat and door...
One measurement to focus on that may not be apparent right away is the height under the steering wheel. I am 6' and have always had an issue with leg room and the big oem wheel in these cars. "you can about set a six pack between seat and door..." - sounds like the voice of experience ... lol