There is only one frame which falls at the widest part of the hull so it has no “bevel.” (This is an angle usually cut on side frames to match the angle of the sides.) The bottom of the long frame piece is 39” long, the width of the bottom of the boat. It needs to be slightly longer to allow for the angle that matches the flare of the sides. Clearly mark the center of this piece. The side pieces are cut long, and trimmed later. The gussets are sandwiched between the bottom frame and the side frame pieces. (see drawing) A scrap piece of 1x2 can be used to temporarilly connect and stabalize the frame while bending the sides. Glue and nail the assembly, with one 1 ˝” SS deck screw in each corner where the frame members overlap. Saw the corners off to allow water to drain under frame. If you like you can add "limber holes" - half round holes in the bottom of the frame, again for water drainage.
Temporary frame approach:
If you are only going to row the center frame is optional, if you do the spacers and inwales to give her a bit more stiffness. The center frame measures 48" across the top, 38" across the bottom and 14 1/2" tall. Use a square to keep it in alignment then screw braces to it. The "W" pattern gives good triangulation. This frame is disposable as the strength of the gunwales are designed to hold the boat's shape.
Here I've tacked the sides frame and transom together in a dry run to test the fit.