MSD distributor 408 stroker

Usually the bushing is put in the block before assembled. Most of the time depending on the press fit they have to be reamed to size for the correct oil clearance. This should be done before the pump and pan are installed,

Also I already had a bronze oil pump drive for a roller cam that was not pinned correct and the shaft was to low in the gear just enough that the Distributor flat didn't reach the slot.

Not all distributors can be used with the gasket. O-ring only on some. The flat doesn't always reach into the slot that far. An issue like you are having can be measured. I have MSD distributors here for parts I can measure and tell you if the shaft is short. Measure from the base flange to the tip of the shaft and post the measurement. Also from the top of the slot in the gear to the top of the distributor hole in the block. There is A fresh motor on the stand in the garage I can also measure for you and post pictures of both measurements.

LA blocks should all have the same measurement no matter the stroke unless the top has been resurfaced for Deck height change for intake fitment. But that would make the shaft to long . Steve