Here's the kicker (and a definite learning lesson):
I started by installing the bottom row first. Prior to starting, I pulled a few signs out of their packaging to measure the width. All three signs measured right at 5-5/16" or 5-21/64". So I did all of my math based on 5-5/16" (see where this is going?).
Long story short, I got to the 39th sign on the bottom row and realized something was off. I held my 3" spacer up, placed the 40th sign up on the wall by hand and sure enough, it was right about 2" past where it should have ended.
I go back and start measuring the signs and some are dead on 5-5/16". Some are 5-11/32". Some are a full 5-3/8". So I don't know why there were the differences in width (router issue? hand-sanded/finished?), but those differences over the span of 40 signs, culminated in the row being a bit shy of 2" too long. I already knew the answer, but explained to my customer and as suspected, I had to redo the bottom row.
There were multiple other issues that made the job more difficult and added entirely too much time to the process, but I'll save you the whining.