This is a measurment I took of the outside diameter of the tips, from the rear. This part is closest to the muffler and not the actual opening. The width is 5.6 inches by 2.9 inches, as measured on a dial caliper. The exhaust opening also measures 5.6 inches wide, but I had trouble determining the actual height. Since the opening has a cut angle and there is a slight flare to the top of the tip that points upward, I don't want to bive you the wrong impression. I'm not sure that the tips I have are original, but I think it might be safe to assume that the measured acceptance for this opening might be 5 5/8" x 3" in standard measurement, as far as a production piece should go. Yes, I do have too much time on my hands to not just give you a simple answer. Sorry.
Measured the rectangular exaust tips on my 70 Roadster and confirmed Bullitt's measurements.
He did give you the simplest answer possible given the angled cut on the end of the tip. Another person might have measured the opening height by measuring top edge to bottom edge, and come up with 4", but the actual height is 3" (standard measurement - as Bullitt put it) with a beveled cut, creating the larger actual opening measurement of 5.6"w x 4"h with the slanted cut.