A few things people haven't mentioned...

You can use a 3/8 drill but it will need to be 120 volt. Cordless drills don't have the torque to spin the pump fast enough to build much pressure.
I like using my heavy duty 1/2" Milwaukee drill. It only spins 800 rpm with a 14 amp motor. The removeable handle that goes over the snout of the drill is real handy.
Go easy on the drill. If you run the drill at full speed and it slams full pressure on the oil pump it will try to rip the drill out of your hands. Spin it up at a moderate speed and allow the drill to take up the load.
I use a piece of 5/16" hex stock as a priming rod. It's dirt cheap. If you take this approach be sure to put it down in the hole for a test fit. Make a note of where the rod passes through the distributor bushing in the block. Wrap some tape around the stock so it doesn't chew up the bushing.

When Andrewh and I prelubed the Imperial it simply wasn't possible to get to the crank well enough to turn the engine over. After we built up oil pressure I jumped the starter relay to turn the engine with the starter while he continued priming with the drill.