Is the engine in the car yet? I'd do it before the engine is in and then again right before you start it. Like dutch said, it's easy to possition the gear using a large screwdriver.(even easier on a BB then a SB)
I primed mine on the engine stand and found a big problem. Better to find it before the engine is in the car. Turn the engine over with a breaker bar while you prime it. Make sure you're getting oil to both rocker assemblies.