Good on using a bore gauge. Don't you mean 3.912"? Do you mean .002/.003" of taper, or just different than the nominal 3.910" bore diameter with no taper?
The Magnums have a reputation for low bore wear, reportedly due to the fuel injection: less fuel excesses washing down the oil on the cylinder walls than in a carb system.
For keeping the original pistons, the next step is to measure ring side clearance to see if the ring grooves are not excessively worn. Measure a ring segment with a mic to get the right height find one that is not much worn from stock spec of 5/64" (.078") , then insert into the groove and check the clearance with a feeler gauge against spec. Worn ring grooves allow the ring to twist up and down excessively and will wear it out quickly. The top groove tends to be the worst.
Have you measured the pistons on the skirts to check diameter? IDK how high or low on the skirt that should be measured for that year piston, but you ought to be able to find the info somewhere or in a FSM.
