I have a couple milled receiver kits from russia (triangle with arrow) in excellent condition and am looking to rebuild one of them. Firing line seems to be the only supplier of milled receivers. which one do i need, the pinned or the threaded. How hard is it to remove the barrel from the cut receiver.
as a side question the original receiver has all the hard parts to machine it would be very easy to mill the back part of the receiver and weld it to the front secton
would this be illegal  I would love to keep the russian markings

your barrel will be a screw type..  
the easiest way to remove the stub is to dremmel a slot on both sides almost down to the threads than split it open with a chisel... or you can just unscrew it  but its in there very tight..

Dont bother welding the stub into a stamped reciever.. yea it can be done but you wind up with a crappy looking ak..  Use the FL reciever  and you will have a classic Ak to be proud of..
you can always have the markings engraved into the Fl reciever...

This is my Russian AKS  on a Fl reciever,,,,

I had FL build mine for me.  It was a Fixed stock Russian kit from Sportsmans guide.  It turned out great.  I would definately recommend them.  Sorry i have no pics available.