Rick's right...you'll just need to turn the
oil pump a little bit so the distrib will slide down over the drive shaft.
You did put the shaft in from the bottom up right? There should be a little metal ring that acts as a flange.....you want that to be on the bottom....not sitting on top of the driveshaft guide hole in the block....or else when you pull the distributor out later on, it will come right on out with it.