If you're not getting the distributor drive tang down in the oil pump slot what could be happening is when the engine is fired the distributor does drop into the oil pump slot which would leave the clamp loose and the distributor free to rotate.

The distributor needs to be seated properly before tightening the clamp.

Sometimes it takes several attempts to get things to line up.

Take note of where the oil pump slot is and turn it with a long screwdriver so the distributor tang will mesh with the oil pump slot.

When you slide the distributor in, the gears will mesh first so you need to make sure they are where you want them vis a vis having some distributor rotation available for final timing & tuning.
Keep in mind the distributor shaft/rotor will turn as the distributor goes in due to the angle on the gears so you have to account for that.

As the distributor goes down the tang should drop into the oil pump slot.
Then the distributor will be fully seated and ready to clamp.