Friend, you can't install the detachable mags without modifications to the basic rifle.
SKS was designed and built with an integral 10-round magazine and that is the only way it was made and used in military service.
As these rifles have arrived here, they either have had the integral magazines pinned to comply with our (incredibly-stupid) laws, or the depth CHOPPED for the same reason.
You would have to replace the entire triggerguard/magwell housing at least..... and I have no idea if this can be done without butchering the stock.
Rifle reloads real fast with Chargers anyway, so that is the best way to go.
Cheapest, too.
Hope this helps.