There are a few ways, but the best way is to install a starter bump switch. You can also turn it by hand with a wrench on the crank nut, but in an F-body that is a pretty tight space to get a wrench. You would have a much easier time if you pulled the fan and shroud... but again it depends on if you want to wire in a switch or remove parts

You could also have a friend in the car bumping the starter with the coil wire unplugged.
Anyway, the more accurate way is to use this method. Starting at the #1 cylinder bump the engine over until the exhaust valve just starts to open. Then you know the intake valve is on its base circle and you can adjust it. Take it to zero lash (spinning the pushrod between your fingers until you feel it get a little resistance) then another half turn sets the preload of the lifters. Bump the engine over again until the intake valve has lifted and starts back closed. When its half way closed, adjust the exhaust the same way. Move on to the next cylinder and do the same.
Be really careful which valve you have open since small block chevys alternate orders, IE, EI, etc.
There are other methods as well that are less accurate but easier to do. If you have trouble with this method, let us know and we'll help you out with another one.