Use the TDC mark to set the distributor where it will start until you get the light on it. You are just literally shooting for TDC as the timing or slightly BTDC.

Easiest way I know to hit the mark the first time is to have a reliable TDC mark, put the intermediate shaft in so that it is at 9 and 3 (straight across) and then put the distributor in. Just make sure that the rotor is pointing to the #1 terminal and it should hit first try if everythign else is in order.

Check and make sure you have a good ground. I've seen engine put back in cars and they crank slow when they have a ground left off or too small a wire. Easy thing to miss in the excitement of a first fire.