Howard,

Cut your frames to the low deck shape.
Then cut additional 1/2 pieces which are the top part of the high deck shape for 3 or 4 frames. Screw the high deck piece on to the frame just at the forward end of the cockpit. Round that frame off to get the curved shape you want.
Take the next frame forward and screw it on the low deck frame.
Then take a batten or just a strip and test fit it from the cockpit frame foward.
Grind off the corner of the next high deck piece until you get a nice curve (both sides - left and right).
Then add in the next frame foward and repeat, until you get everything faired out.

My reason for having you keep the low deck upper frames is that if you make a mistake with the high deck add on, you can make a new one without taking the entire boat spine and frames apart.
You can even just make the entire hull stripped and stapled to the tool before you flip it up to do the deck. Now you have something really stable.
Be sure to mark a waterline on each original frame and the same waterline on the upper halfs so you can accurately align the upper high deck add ons.
You can even make the entire deck, take a look at the lines and start over with just the deck if you don't like it.
I had to remake my deck on a kayak because I messed up the height of one frame and didn't notice it until I could see the entire deck line.

The other way is to learn to make a 3D computer model and modify the lines anyway you want - then print out the new sections. This may be a lots more time and effort than what I talked about above.

Good luck.