Mark, you're a bit off track, but hey, it's fiberglass. It's hard to make a mistake that can't be fixed.
From a previous post:
...trim either or both to an imaginary line which exactly bisects a 13" circle.
That means trim before
clecoing them together.
Start by marking and trimming the upper
cowl. Align a straightedge as shown; 6.5" at the center (half the spinner diameter), while maintaining exactly equal height on the two inlets. Mark at the four red lines. Trim just outside the line (maybe 1/16"), which leaves some room to clean up the edge with a long sanding board. At the outboard cuts, don't try to trim back along the sides of the cowl just yet. Just trim the leading edge of the inlet opening:
Now trial fit to the lower cowl, on the floor, with a spacer stick or two at the rear, and the long sides overlapped. If you're lucky, the lower cowl joggles will match the freshly cut upper cowl. By lucky, I mean won-the-lottery-lucky; you'll probably need to trim or fill to get a match.
BTW, if the joggles need a lot of work, it's easier to just cut them off, trim the lower to match the upper, then add new joggles. To do that, set up and trim the lower cowl nose pretty much as you did the upper. Cleco a temporary scrap joggle any where needed. Plenty of opportunity to fill the cleco holes later in the finishing process.
With the nose trimmed to a pretty good match, align the cowl on the airplane to trim the rear edge and the sides. Attach a steel or aluminum tab to the forward baffle attach points (1/4" screw) above each front cylinder. These support the nose of the upper cowl. The rear is supported by overlapping the boot cowl. Draw a centerline on the top rear center of the cowl. Draw a matching centerline on the top of the boot cowl. Adjust the length of the front supports so the cowl nose is dead behind the spinner and perfectly level. Install a block to set the spinner gap. Align the centerlines at the rear of the cowl. Now trim the rear edge of the cowl so it sits on your hinge line or camloc tabs. The upper cowl is now where it belongs.
Raise the lower cowl into position, install the two front clecos, and run a strap around the halves. Trim the rear edge. Now trim the sides. Start by drawing a level line back from the front of the cowl where you had already established a bisect line in the very beginnning. Trim both
halves to this line.