I have seen on the sludged motors that the oil pump shaft won't want to come out, you have to really pry it out hard. it is from carbon and deposit build up on the shaft and oil pump drive bushing. I assume you took the block to a maghine shop and had the block hot tanked and clean? also if using the oil pump shaft from the sludge motor, pull it out and clean it really good on a wire wheel bench grinder. If the motor wasn't hot tanked, get an engine brush and clean the oil pump shaft bushing. Also clean the oil pump shaft where the dizzy shaft goes into it and clean the dizzy drive tip.