Since this is the legal forum, I'll give you the legal answer (obviously, you can do whatever you want).
An SKS that was in the country before 1990 is not subject to the 1989 Import Ban nor the 1994 Krime Bill/Assault Weapon Ban. The catch is: you have to have some way of PROVING that the gun was in the country before the Import Ban went into effect.
Now, before you get upset, having proof is NOT *specifically listed* as a requirement, so some will argue that "it's up to the government to prove it wasn't." Unfortunately, due to the way the law was written, it doesn't work that way in court. The law is written to ban ALL imported guns in "non-sporting" configuration (as defined above). There *is* an exemption for those rifles imported before the ban went into effect in 1990, but YOU have to have evidence, of a type submittable in court, to prove that the rifle was in the country. Lack of proof could get you convicted of manufacturing an illegal gun.
I know that there are people who don't agree with this assessment, but you should note that the LAWYERS on this group all do agree. They've been to law school and in court, and know how the law works.
Anyway, assuming you had, say, the original bill of sale from your FFL dated 1988, or some comparable dated document or other suitable proof, then you would be free to configure your SKS into any non-NFA configuration you like.
-Troy