My MSD distributor will only go in this far, it’s same size as the original one and I have turned over the engine to try to seat it. It feels like it is hitting the bottom of block.
Usually jiggling it and/or lightly tapping it while pressing down usually gets it to seat.
If that doesn't work, I've used folded sandpaper, or a small file, and lightly clearance the inside of the hex on the distributor to get it to slide on easier. You can use a 1/4" socket drive or the like to test the fit.