Distributor won't sit down all the way on my 327

[chewychevy67]

So I'm getting to the final parts of my engine before I can run it. Sat my distributer in last night (tried 2 different ones) and it won't sit doen all the way. It's about 1/8" or 0.010 from the intake. It is down into the cam gear as it should be but must be bottoming out on the oil pump shaft. Block wasn't decked but the heads were shaved. All gm parts here. Is this common after machining heads ? Anyone have a good procedure for shimming and making sure the gears mesh properly? Melling oil pump and shaft.