First of all, rotate the engine so that #1 cylinder is at TDC. There are a few different ways to determine TDC. I usually stick a long piece of vacuum hose in the spark plug hole, and with the valve cover off, rotate the engine by hand so that both the intake and exhaust valves are closed and the vacuum hose is almost all the way out.
Get out your handy shop manual and determine which spark plug wire goes to the number one cylinder on the distributor cap. This is where you should try to get your rotor to point toward on the distributor cap after it's in the engine. You might have to have the rotor a little counter clockwise of your desired position as it will rotate clockwise a little when it engages the cam shaft gear. Also, the vacuum advance should be pointing straight forward when the distributor is in.
You may also find that you'll have to rotate your oil pump shaft to get the distributor to fall into place. Just use a socket on an extension to rotate it a little bit.