[Tommy]

Chris -
My EM car was built around 1995 with a 90" wheelbase and a 351 Windsor engine. The body is currently off the chassis. The best reference points I could find for the exhaust openings were the forward most surfaces of the footwell boxes and the bottom most edges of the body immediately next to the openings. Here are the dimensions on my car.
Driver Side : The opening is 5 1/2 inches wide by 5 inches high. The height was measured from the bottom edge of the body around the contour of the body using a flexible tape measure. The rear most edge of the opening is exactly flush with the forward most surface of the driver's footwell box. The corners of the opening are radiused about 1/2 inch.
Passenger Side : The opening is 5 3/4 inches wide by 5 inches high. The rear most edge of the opening is about 3/8 inch aft of the forward most surface of the passenger's footwell box. The corners are radiused.

One more observation from the guy who didn't build this car: The footwell boxes limit the aft position of the exhaust pipes. I see no reason why the opening should extend aft of the forward surface of the footwell box. I suspect the reason the opening on the passenger side of my car is larger than the opening on the driver side is because it was inadvertently placed too far back and then enlarged to compensate.