If possible use the stock hinges and latch to secure it. You probably wont be able to attach them directly to the new seat, thats where you have to be creative. Since the fiberglass style seats are not that strong, you might have to create some kind of support directly under the part your ass will sit on.Think wood or metal sheet/plate for a base pan. Attach the stock hinge and latch to this base pan. Then fasten the seat to the wood/metal base pan. I would recommend using cardboard as a template to transfer the stock mounting positions and finding good locations to fasten the seat....and Alakazam your done!
Good luck!
Paul Z.