I am restoring a 23 runabout and after redoing the wood and body working, I find the large gapes between the body and turtledeck. The fit at the top is tight but then the gape gets larger to about 5/16 at the curve. Anyone have this problem and if so, how did they solve that.
take out the horizontal stiffener across the inside/back of the back panel. Try to push the trunk closer to the body, fasten, then reinstall the stiffener (may have to reshape it some).
I kept the turtle deck tight to the body during rewooding and had a perfect fit. For some reason, removed it when I made the two rear curved pieces of wood. They unfortunately "fixed" the curve, and thus the slight bow across the rear panel, which wanted to kick the trunk back a little JUST like what you're experiencing...I sure wish I'd left it bolted in place when I made those two last pieces.
Doing what I explained allowed me to pull the trunk up tight.
You will find that most roadsters that have been rewooded have a really poor fit in this area.
Best of luck