You could reinforce the hinge areas by sliding some long stiff metal tubing into the fiberglass channel that the original hood hinge brackets are embedded into, and then set it in place with some fiberglass epoxy poured into the channel. I'd drill some holes in the stiffening tubing so the epoxy gets into the tubing as well, not just around it. Repair any issues with the hood hinge bolt threads first, and insert lubed bolts in the threads before pouring in the epoxy. You'd need to jig the hood so it remains straight while the epoxy cures. You can gain access to the channels by opening the very ends of the channels at the back of the hood, where the channels end, by the windshield, then epoxy back the cut pieces. There's no going back after this repair, so make sure any bend due to the flex at the old hinge end point is removed and the hood lines up with the line at the top of the fender.
Not sure as far as the underside finishing goes - whether they tinted the gel coat, dusted the underside with flat black, or just 40+ years of dirt and heat altered the color.
Most guys removed the hinges and pinned them at the rear - pretty common to see them with 4 pins.
