The SILVERSIDES was a modified STURGEON class, being 15 feet longer. The Scale Shipyard has a 1/72 scale fiberglass hull and resin fittings set for the ARCHERFISH (SSN 678) -
http://scaleshipyard.com/Catalog%20Pages/submarines.html#SUB-12. The hull is 50 & 3/8" long and 5 & 1/2" beam. The Scale Shipyard also offers STURGEON class hulls in 1/96, 1/72, and 1/48 scale. You'll need to supply the prop with any of the SSY hull kits.
Thor Design has a fiberglass kit of the STURGEON in 1/96 scale,
http://www.thordesign.com/sturgeon.html. However they are currently retooling their mold, so a kit won't be available until late 2006. Thor Design kits come with a prop, cast metal fittings (periscopes, antenna, deck fittings, etc) and photo etched stainless steel MBT vents.