Not fast progress this morning. Rather than using hardwood planned to exactly 1.438" or 1 7/16" I decided to use nuts and washers as spacers since they will be more dimensionally stable. But I spent the bulk of my 2 hrs looking for appropriate sized washers to get the dimensions just right. In the end, it came out pretty good. I then cleco'd the assemble to the forward floor to make sure the rivet holes lined up as a double check.
Also the 1.438 doesn't seem to match the spars they are closer to 1.5 on my calipers so I when with 1.52 on th spacers. I will double check this again before I rivet anything together. Hardware list:
1/4" x 3.5 bolts (8) 5/16" nuts (32) Washers assorted sizes and quantities 1/4" nuts (8)
I went slightly larger than called for 1.52 vs 1.438, I may readjust that down before I rivet.