If the gun is eligible to be imported in it's current format - a 5-round mag-tube, and it is modified using domestic (US mfg'd) parts, it isn't in violation. JMHO and IANAL.
You should be wary of state level restrictions, however. While the federal act "sunsetted", many of the more restrictive gun-control states kept versions, or their own stricter versions, of the AWB in place. Check firearms law for your specific state. Fortunately (for me) NJ patterned theirs after the federal AWB and it only applies to semi-autos. Traded my "tacti-cool" Rem 1100 for a SuperNova last week. Mag extension goes on tonight.
Good luck.
Adios,
Pizza Bob