I take it the cleaning rod is out,the Yugo's have been in cosmoline for quiet some time the stuff can be like glue when it comes to separating the stock from the receiver.
Best way to separate it is to warm it up in the hot sun a little if possible then place the rifle receiver down and prop the back of the stock up slightly and use a wooden dowel and hammer through the trigger group hole and gently tap it out.
Don't use gas to remove cosmoline to explosive K1 Kerosene or plain old HOT soapy water works fine. Here is a good site on the SKS lots of useful info. I would clean my stock with mineral spirits,you can also use low heat to bake the cosmo out of the wood I don't recommend putting a wooded stock in a dishwasher or any water for that matter you may have some fit issued from the stock swelling up getting the receiver and trigger group back in. Also always make sure you cock the rifle let the bolt go back forward put the safety on before removing the trigger group makes getting it back in a whole lot easier.
Collecting and Shooting the SKS Carbine - HISTORY