Before you start mods on the selector stop, make sure that the front trunion is in the proper location.
On my tantal build (ak74 style), I had to file the receiver in front of the selector stop.
This is what you do
Get some small bolts and nuts that will fit the triggerguard.
Bolt the trigger guard in place.
The put a mag in the mag well.
Slide the front trunion in so it is TIGHT against the mag.
put a C-clamp on the reciever section above the front trunion.
Once the clamp is tight, check the mag for looseness.
If the mag is loose then file the receiver (right in front of the selector).
Once you file the receiver you will need to loosen the clamp and slide the trunion back some more.
If you are using a matching #'s kit, and you are pretty sure the trigger guard came from that kit, you should not have to file the selector stop if you follow the above steps.