The bels should be mounted into the frame or some structural metal. The outboard belt is easy enough, you can bolt it into the frame rail. The inboard is not so easy. There is nothing there but fiberglass. When I had my safety cage constructed the chassis shop installed a metal brace off the frame to mount the belt. In lieu of that, use the largest metal plate you can fit under there as a reinforcement to the fiberglass.

In retrofitting a defroster system, you’d have to remove the windshied to cut holes in the cowl. Unless you plan to drive this in the winter, don’t bother. I did drive mine through several winters and found that a small 12volt heater from Harbor Freight that I could place on the cowl worked just fine.

Windshield washers, you’re on your own. Never had them. The original cars never had them. Personally, I don’t think you need them.

Good luck with your purchase.