There is no problem with widening the magwell to fit the original magazine, though I'm surprised that you're having a problem with it. I recall that the original mag is narrower than the Tapco.
In any case the magwell doesn't really do anything to support either magazine, the SKS system is overly secure so there's nothing to worry about, go ahead and dremel it out.
Coach is right, putting on the Intrafuse puts you in to 922r compliance and the Tapco magazine is a very easy way to get your parts count up. About the only other way to be 922r compliant and still use the original magazine is to install Murray's trigger parts.
Laws are laws. Like it or not, violating 922r makes you an illegal gun owner, or the owner of an illegal gun, depending on how your lawyer wants to defend you. Personally I would never advocate that anyone should willingly and knowingly own an illegal firearm.