The SKS is not a precision made rifle, particularly since they've been made in numerous plants in numerous countries. It's not reasonable to think that a single stock is going to fit them all perfectly.
Some dremel or file work should solve your problem and I think this is a common occurrence.
The area most likely to need some cutting is at the rear of the stock where the trigger post/lock is. Cut a groove in the stock that will give that post enough clearance to fit in. The whole assembly should fit tight, but if it's too tight the safety may not disengage. If that happens cut away a little bit more.
Do not cut at the front of the stock unless there's no option. The receiver assembly end cap should fit over the stock forend properly, but is not a deal breaker if it doesn't, if there's slop up front it can feel a bit loose and that does suck a little. So put the endcap over the forend first and then push the rear of the receiver in to the stock. If it hangs up, start cutting at that point, just a little bit at a time.