bummer. are you sure it will be an issue? how far out are they?
also how hard are they and what material? proper receivers are springy and pop back so you'd have to go beyond a bit to get it where you want it.
if you have a heavy steel table you can set a spacer on it could be a thin dowel or flat stock, lay the receiver on top of it perpendicular to it. and clamp down on the front and back of the receiver. you might use a LITTLE heat. I would do this rather than trying to pinch the rails together. go a little at a time of course.
if its heat treated properly it shouldn't crack. until you went way to far. but since its warped maybe the quench isn't all they got wrong.
before all this, will they replace them? these aren't serialized shouldn't be a hassle.
fcg holes, recreator blanks or the guys in Colorado that make the cool jigs make a flat drilling fixture. it wont fit on the milled receiver as is because the bottom is thicker where the front of the t.g. gets riveted. but the one I have the sides unbolt.
you can either put spacers between the sides and bottom to get the height you need or you can just take it apart and use it in pieces.
I take extra fcg pins and turn them down to use as dowels to hold the jig in place in the gcg holes while I make the holes for the selector.
if you have an akm receiver you can take all the dimensions from it as well. coming off the top of the receivers it will be the same.