tripwirenh,
These old lunds are great and they handle really well in the water. I don't think you will be disappointed. To answer your questions:
1. I used 1 1/2 by 1/8 aluminum angle. I initially used the 6061 for the sole because it is better for structural support, but to butt the joints together well, i had to grind/file the ends to match the curvature. For the framing on the seats/boxes, I used 6063. Much easier to work with since all angles are 90 degrees. I bought my stuff from https://www.speedymetals.com and https://www.onlinemetals.com. Not sure if they are the best prices now, but they were when I researched during my build. One thing to note, buying online has good costs, but you have to factor in shipping especially for pieces that are larger than 3 or 4 feet long. But the nice thing about these sites is that they do occasionally have 20-30 percent off sales. Also, if you live near a metal wholesaler, then picking up at will call will save on shipping.
2. I used both aluminum pop rivets and stainless steel screws and construction screws. I riveted the 1/2 inch plywood sole and the side panels to the frames, and to join angle pieces together (that is anywhere aluminum joined aluminum). I used stainless steel screws with lock washers (for extra security) to anchor the aluminum angle that was attached to the sole for the boxes and the rear seat. I also used stainless steel screws for attaching the box tops and the rear bench top to the aluminum angle because I couldn't find pop rivets with the proper shaft length. For the front deck which is joined onto the wood bench seat top, I used #10 or #12 construction screws.
3. You have it right on the hatch lip. I simply glued either a 1x2 or a 1x3. Can't remember the size. Definitely a wider board will give more surface area for the glue to stick.
4. One last thing, for the tackle locker, I used 1x1x1/8 6063 aluminum angle. I think i used a 3/4x1/8 bar as a rear "stop" to keep the tackle utility boxes flush with each other and to keep them from moving around.
I will say, I'm not sure how economical it really was. Definitely cheaper than getting a new boat or even a used boat with the same features. In the end, it's really not about economics because once you get started, you are always thinking of something new you can add. The nice thing about doing your own mod, you can customize it the way you want. Also, you have to account for accessories. Just the new seats and their hardware must have run about $400 and I got the seats for 30 percent off.
One last thing, I'm sure the boat will run fine with the 20hp, but when you add all the stuff onto the boat, you will likely add at least 100-150 lbs to the boat. While the boat is a lot more stable in rougher waters with the mod because of weight, I definitely lost some top end on speed. With my '67 Johnson 33hp (equivalent to a modern 25-28hp), I was getting a top speed about 26-27mph with the first mod, but it went down to about 23-24mph on the top end with this mod (25mph if I am going with the current/wind).