Two options as I see it: a) Modify the tailpost to meet the vertical sides, OR b) Build and set the tailpost properly, then let the lower longeron "adjust" itself over the last couple of bays.h Personally, I don't think the ply will have any difficulty in following the slight change in contour as it will be conical, and not a compound curve. You can test this while on the build table. Install your upper spreaders, then before permantly installing the lower spreaders use blocks on the table to spring the longeron against. Fit a sheet of ply up against the side, hold in place with spring clamps and look for any areas that don't lay flat. My .02, Mike B
