well if your SKS is a beater and a collection of mismatched parts, you could modify it so that it ejects sideways rather than slightly upwards (this involves removing a wee little bit of metal on the right side of the receiver). But if you don't want to damage it in any way, then, indeed as others mentioned the magwedge should do the trick.
As far as scoping it to shoot 300 yards - well, some would argue that that's ambitious, somewhat, but hey, I never look down on ambitious folks. And the rigidity of the magwedge rail can vary from gun to gun; so there's a bit of an advice, expect some tinkering to make your platform rigid and reliable.
You will still want to see your target, and you will want a scope that holds zero (I assume) so, don't look at budget, lower end scopes (NcSTAR etc). Do yourself a favor and invest in a reasonable-quality scope. Don't need to spend $40,000 but, let's say, a used scope which costs as much as your SKS ($200 to $300) should get you a decent (ish) bushnell or Vortex or other similarly reputable scope.
I guess others will have different opinions but, that's what I can offer on this Sunday evening
