Nice boat Jake! A friend of ours has pretty much the exact model. It is bomber. When we started rafting about 8 years ago, we purchased an old 16' Riken bucket boat with a galvanized frame similar to what you are thinking of making. That rig served us very well.
The first thing I did was add some internal boat sealant:
This stuff is amazing and I highly recommend it. After applying that we never had to top off our boat with air on trips and it kept air better than some of our friends newer boats.
My other recommendation would be to replace the plastic military valves if your boat has them. The plastic/nylon in the valves oxidizes over time and can fail and snap off into the raft tubes. It isn't too replace these as long as the existing boot is in good shape. However, I wouldn't want to do it on a trip.
Finally, get a case of 303 protectant and use frequently.
I think making your own frame is a worthwhile endeavor. I didn't mind the weight of my galavanized frame in the water as much as I did loading and unloading it. Keep in mind with a bucket boat you'll need to install it at the ramp every time since you won't be able to keep it rigged on the trailer. As long as you have a strong buddy or two, not a problem.