if you're sure it's just the bottom plate, order one of these
http://www.wolfsburgwest.com/cart/DetailsL...m?ID=111801171Cand cut out the old and weld in the new.
however, you may discover that there is alot more damage to the heater channel when you open it up. if your car is a west coast car, i'm guessing there will be alot more rust than is visibely noticeable.
pull out the back seat, take off the drivers rear panel and pull up some carpet. how is that corner down on the floor where the rear crossmember meets the end (rear) of the heater channel? how about the frontwheel well? look at the front firewall where the heater channel begins and look for signs of rock chips, holes, p/o repairs, etc.
i'm not trying to scare you my friend. just warning you that the heater channel in a west coast bug can be a world of ugliness the deeper you dig.