I ran into the same thing with a low mile 318 when I swapped it to electronic ignition.
Careful measurements revealed that the distributor was bottoming out before the upper round plate could sit flat on the block.
So I used a length of PVC pipe and with a large miter saw for wood, I made a series of wafer thin spacers.
I test fit the thinnest first and then worked up. In my case one about 1/8" thick worked.
The O-ring still seals and the clamp keeps it from rotating loose now.
Many happy miles since.