Have you checked to see if the tab on the end of the dist. shaft is bottoming out in the slot on the oil pump shaft? I'm assuming the Mallory dist. was in the old engine & so you know it should fit properly.
Yes the Mallory was in the old engine. I’d hate to have to remove the timing cover but it’s the only way to see what the issue is.
Use a little emery cloth to dress down the edges maybe there is a slight burr on edge. It looked like you had an old distributor that was why I was saying to clean up the parts, But would not hurt to do it anyways. Spray the outer edge of dist with silicone spray this will help with sliding in easier. I got to say it is tight it took me a couple tries. For heck of it just see if you can get the dist to seat without trying to get to #1 see if it will go in at #6 on other side of engine just to see if it will go in there. If it does twist it as far as it will go both ways and maybe this will help get it loosened up and if it fits in there like it is supposed to take it out and see if it goes where it belongs then at #1.
You can check if the dist. shaft is bottoming out on the oil pump shaft w/o removal - A little grease or something on there would show contact.
Before you pull timing cover, stick a piece of shim stock or use a straightened coat hanger to measure depth. Or pull oil pump cover and drop drive gear. Something is fubar.
Well when I got home from work today I did as you suggested. I measured with a thin piece of flat stock that I had laying around (hence why I have no room in my garage) and I couldn’t find a reason why it wouldn’t fit. So I threw caution into the wind and put the bolt with the hold down and put slight pressure on it. Lo and behold it fell in the rest of the way. I looked for witness marks of why it wouldn’t go in couldn’t find a thing. Pulled it out put in back in dropped right in. I should have just hit it with a hammer in the first place. Lol
Come to think of it just now, I had a similar issue with my MSD distributor and new TA 350 cover, the fit was pretty tight. That was a few years ago, probably why I didn’t remember.
I did want to force it in although a short distance thinking it would get stuck. Or worse break something off in the process, then search to find it.