That frame is an accident waiting to happen. Ditch the 2 X 2's and replace with 2 X 6 or 2 X 8's. Move the bolts so they are opposed, the two on the outside of the tubes should be mounted so they are shortest longitudinal dimension, the two inboard should be longer. That gives you an "X" which is far stronger than having both in-line (in the same plane). Yes, plywood helps, maybe you want to sandwich a couple of 3/4" pieces instead of going to dimensional lumber (2 X 6 or 2 X
But you need to get rid of that 2 X 2 that runs the length of the frame...
You can countersink the nuts and washers or cut donuts out of other pieces of wood to make them safe to walk on/around.
Remember yr boat is almost 30 years old regardless of its appearance. Congrats on the score, but keep an eye on EVERYTHING. Every time you inflate it should be baffle by baffle (chamber by chamber) to semisoft (1.5# or so, you can press and deflect with your thumb). Then top off to 2 or 2.5# - you will be able to feel the difference.
I hope I'm wrong, but I'll bet a case of beer that if you use it 15 or more days per year you should count on replacing in 3-4 years. I've hauled a dozen old hypalon boats to the dump in the last couple of years, including 2 of my own, and I babied them (stored indoors, inflated as often as possible). When you roll it up and hear what sounds like corn flakes crushing, it's soon done... that's the coating inside the tubes giving up.