mine did have the collar that keep it from pulling real far from the pump the end of the shaft was on the pump leaning sideways preventing the distributor from going all the way in

 

I was wondering about the retainer as well; the pump shafts on all my oil all have a locking washer that prevents the shaft from being pulled out from above.

You might try this grabber tool and get lucky:

https://www.harborfreight.com/24-inc...ool-94162.html

I once dropped an oil pan plug into a waste oil container, and managed to pull it out with one of these, but I had a lot of room to manipulate the grabber.

 

Those little retainers have a tendency to move if you get the end of the pump drive stuck in the dizzy. I pulled apart a 250k mile 351w that had the shaft stuck in the dizzy. Pulled the whole thing out the top when I was removing it for a rebuild and the little clip rode down the shaft and into the pan. Would have been a big problem if I wasn't already pulling it apart. And then, because I'm a genius, I forgot to put the pump drive in until I already had the motor fully assembled except dropping the new dizzy in. I put a dab of grease into the end of the dizzy to create a "suction" effect so it wouldn't drop the shaft as I put it all into the motor. Chances are though, if he dropped the shaft into the motor, it's probably laying at the bottom of the pan now.

 

I now when pulling the distributor I rock the rotor back and forth as I pry slowly pulling it out

I think that when I was priming the engine I pulled the shaft out with my drill

 

posted this a while back, might help...
I just went thru a very strange situation. I have a 302/5.0 crate, not sure of the year (85 up) I had a Unalite go bad and decided to go with a Pertronix flamethrower. When I went to install the distributor it wouldn’t fit. It felt like the gears were starting to mesh but it wouldn’t go down in the block. I thought the oil pump drive shift was not lining up ( it wasn’t) I put grease and the shaft to hold in place, still no luck. After calling the tech line and more tries, no luck. I then measured everything on both distributors, all was the same EXCEPT the diameter of the distributor shaft( Mallory 0.4490 Pertronix 0.4520 ) called tech again with these measurements, they said it should fit and the MDS that fit that engine measures 0.50. So I then thought measure the block opening for the oil pump drive shaft. The only way I have to measure it was with drill bits (because the hole being 4” in the block) I found the biggest bit that would fit and measured it with calipers, 0.4510. I had to shave it down to .4490 and it fit like a dream.
I haven’t talked to anyone else that has experienced this. Maybe someone on the forum has, if so I’d like to know what year the engine is please.

 

I swapped out my distributor for an HEI style found the easiest way to get it in was put grease in the end of distributor and pick the oil pump rod with a magnet. The grease held the rod into distributor so it could be lowered into place. it took a couple of tries but it worked pretty good for me