On a pram with those general dimensions, your planking should be 1/4" not 1/2", though 3/8" on the bottom would provide some security in beaching. The framing of structural components (frames, etc.) don't need anything near the weight of a 2" x 4". In fact, if well thought out the boat can be nearly frameless (taped seams), with just a thwart or two, some chine battens and the two transoms providing all the stiffening necessary. If you elect to build ply over frames, I'd use 1" x 2" on close enough centers to keep the panels from flexing excessively, say about 16".
I wouldn't rely on spray on goo in a can for water proofing, but would use polysulfide with the ply over frame or epoxy on the tape and seam version. There are lots of these little pram/tender plans available (I have a few to offer too). Most now use the tape and seam construction methods, which create a strong, water tight little craft. Ply over frames is a more traditional method, with less goo factor, but more wood working.