Are you trying to slide it in from the top? It needs to go in from the bottom with the oil pump.
If thats not feasible now, some have removed the stop ring and siliconed it to the bottom of the distributor then slid it in, probably not the best way, and you risk it falling off and into the pan.
If at all possible, drop the pan and put it in the correct way.
do it the right way and be done with it.........the person that had my car before me just drop it in and when i pulled the dizzy the shaft come out of the oil pump but didnt fall in the oil pan right away and then when i started it it bent it to hell and it took me forever to figured out wy i had no oil press.
i had a cherry picker mad it easier for me. i took the engine mount nuts all the way off and jacked the engine as far as i could. droped the oil pan(wont come all the way out) loosened the oil pump and wiggled the shaft to where its supose to be. took me a few hours cause almost every oil pan bolt was a diff size