Hi Charles.
When you sand the top cowl, be sure to use a very coarse sandpaper, like 50 or 60 grit. to ensure bonding.
Before you sand the windshield, lay about 8-10 layers of masking tape on the windshield, on top of each other, along the line that the fiberglass will come up to, to prevent scratching that area.
That said, it's a good idea to fit the black vinyl dash cover first, before you permanently install the windshield. Lay the windshield in place and mark the outer skin against the windshield and cut the vinyl to that line and glue it down.
Best,