OK, so I've had a chance to look at the plans and think about the question of scantlings for the boat. First off, I am not a professional boat designer. This advice is worth at least 50 times what I'm charging you for it, you take it at your own risk. If you really want to be sure of what you're getting, contact CLC and use their consulting design service. There you get some truly excellent and inspired professional designers who can help you out. Of course, they'll charge you, but that's how they feed themselves.
Next, there's a disagreement in the dimensions. The length at the top of the runner is 9'-10-1/2", but if you add up the dimensions at the bottom of the runner you get 10'-10-3/4" and the text refers to a 6'x10' top. Based on all that I'm assuming that the 9'-10-1/2" number is the correct one.
I'm also assuming that the boat will draw 3" so the total displacement comes out to be 600 lbs. That means that everything - structure, propulsion, furniture, people, beer, fishing rods, etc. - combined needs to weigh 600 lbs. Every 200 lbs above that takes another inch of depth. So, for example, if you go to 1/2 ton of displacement you'll draw 7 inches, not 3.
That is one heavy boat as drawn. Assuming DIY store mixed conifer plywood, the framework, a 6'x10' deck and the skinning for the pontoons is 430 lbs, leaving only 170 lbs for all the other stuff mentioned above.
You can immediately reduce that to 290 lbs by using Okoume plywood, like the kind sold here by CLC. That will leave you 310 lbs for everything else. Note that these weights do not include paint, fasteners, additional wood for chines, etc. This is one heavy boat.
The next big weight reduction would be by using less wood. This is a plywood scow, so it will never go fast. Unless you're extremely unlucky or extremely foolish, it will never operate in rough water, either. The most it should have to stand up to is the wake from an inconsiderate motor boater and grounding on the bottom. Because of this you don't need such thick wood. If it was my boat, I'd use 3/8" plywood for the frame, 1/4" for the skin and 1/2" for the deck. That would give 250 lbs in pine and 170 lbs in Okoume.
If the thinner wood makes you uncomfortable, you can add lightweight strength by putting a sheet of 1/4" plywood under the connecting frames between the pontoons. That will form a box girder with the deck that will hold the pontoons together for only 13 lbs pine and 9 lbs Okoume.
The other thing you could do is run a piece of 3/8" plywood on its edge down the center of each pontoon bottom. Make it the same height as the bottom frame and use nesting notches to lock them together. That will make the bottom into a t-beam that will strongly resist flexing front-to-back for only 3 lbs in pine and 2 in Okoume.
To summarize, weights are:
original - 430 lbs pine, 290 lbs Okoume
reduced - 250 lbs pine, 170 lbs Okoume
extra strength - 266 lbs pine, 181 lbs Okoume
Payloads are:
original - 170 lbs pine, 310 lbs Okoume
reduced - 350 lbs pine, 430 lbs Okoume
extra strength - 334 lbs pine, 419 lbs Okoume
That's still pretty heavy, but that's because you aren't getting any help from the shape. Those slab sides are easy to cut and build, but they make no real contribution to stiffness. If you curved the sides toward each other and had rocker on the bottom (both ends curve up), you could use thinner sides since the opposing curves would provide stiffness. But that would make layout and assembly more difficult.
I've also got a few thoughts on construction. Even the really thick plywood doesn't have good fastening area, especially since the fasteners would have to go into endgrain. If you want to build it plywood on frame, you'll have to add chines along the edges to hold the nails or screws.
If you build it stitch and glue, then you won't need chines. You'll be using fiberglass taped fillets to join the pieces. The glass tape will bond to the wood with lots of surface area and the fillets will support and smooth the bend.
Speaking of fiberglass, if it was my boat I'd put a layer of 4 oz cloth on all the surfaces (inside and out) except the top of the deck. For the deck I'd use a single layer of 6 oz glass. On the bottom, once the boat is assembled, I'd add a layer of 6 oz glass over the 4 oz that was already there, being sure to run it up 2" on the sides, back and front. That kind of glassing schedule would seal the wood against water, prevent any leaks at the joints and protect against abrasion. Since the boat is slab-sided, all the glass, except on the bottom of the pontoons, can be put on after the pieces are cut but before they are assembled. That lets you work flat on a table.
I would not put foam in the pontoons. That is just extra weight for nothing. With the construction described above it'll never leak. Instead, you could put hatches in the deck and use the pontoons for storage. Just keep the load balanced and use leak-proof hatches.
Finally, a word about balance. Every 100 lbs on one side of the boat that's not on the other will make that side go down an inch or so, until the deck tilts to where the loose weight falls in. One person walking around will make the boat wobble a bit, but not too much. Two people could start making it more interesting. This kind of boat has great initial stability, but lousy secondary stability. It'll handle some decent imbalances, but once it goes over it's not coming back. So keep the crew towards the middle laterally. Front-back you have a lot more leeway. What you really want to avoid is anyone standing on the outside front of a pontoon. make that area unattractive for loiterers.
Hope this helps and let us know how it turns out,
Laszlo