You do not have to mod the SKS to use the hi cap Tapco mags as long as you dont mind changing the mag with the bolt open.
You can order a new bolt from Tapco that already has the mod done so you can change mags with the bolt closed.
You can still use the standard 10 round mag with this mod.
The Tapco stock needs to be modified for the SKS that uses the Spike bayonet.
Im gonna buy the SKS this afternoon. I already called my guy to have him put it in the vault for me.