Thanks for the reply guys. Yes 84 inches is correct. The opening measures 13.750" wide (at the middle of the trapezoid) and 6.125" high.
My partner and I were looking at how we could cut and raise the bottom about 1.125" (due to the exhaust) but that leaves us with a 68" opening - still very big. I love the idea of different size openings as Dan made but I don't think we are up for that.
Currently we are thinking we can raise the bottom an inch or so and extend the cowling aft a few inches to take advantage of the ramp to further close the exit area.
Hopefully my photo of the exit area will display.
Thanks again for the help!