Today I pilot drilled the piano hinges to attach the bottom cowling. Then positioned, drilled, prepped, and riveted to the forward fuselage with the barrel and opposite hinge side protruding outwards. This is to have some slight distance between the fiberglass and fuse skin to prevent abraision from vibration. Lubed the hinges and scotch-brited the pins for smooth operation. Clamped the cowl in place and positioned for best fit and top parallel to engine centerline. Clamped some 1/4" scrap to the spinner backplate to set clearance between cowling and backplate. With some additional trimming I got it about right I think. Then I backdrilled the pilot holes in the forward piano hinge into the fiberglass and cleco'd. Expanded for A4 holes and cleco'd. With bungee support removed and the cowl hanging on its own, the backplate clearance increased to 3/8", which I'm still calling good. I'm surprised how stiff just those clecos make the bottom, just hanging there attached at the back only.
Overall, I'm pleased so far. Still have to locate and drill the exhaust pipe hole.