..................Comments and suggestions welcome as I try to hone in on the shape of this thing....... Tom
Tom, this has worked for me in the past as far as getting these smaller bodied cars to look right.
It looks like your headed in the right direction with the body ahead of the rear axle, including the roof. For the remaining body behind the "C" pillars do this....
Find or otherwise create a flat/level plane from the center of the rear axle rearward...if you measure at an angle ,your dimensions will be off. Hopefully you have a set of digital calipers or something that can measure in Millimeters going on the theory that 1 mm is equal to 1" in 1:25 ......measure back 40mm from the center of the rear axle and make a mark. The 40mm mark is simply a starting point for determining if its too long or too short or just right....for me 40mm back from the center of the rear axle has been pretty much spot on ('79 Mustang, '78 MustangII Late Model builds as examples) for 108" w/b leaf spring cars like Ed Sanger's
The fallowing is from my Fad Fiberglass stock car body catalog from 1980....this is reference to the 1980 Mustang which is pretty much the same size as the Monza your working on.....
"....front end was extended 9" leaving the wheel wells in the quaterpanle in stock location." The extended portion was the body(fenders) between the engine fire wall and the back side of the front wheel wells, including lengthening the hood.