I am trying to configure Tor bridges that will use only USA exit Relays. 
Is this possible? I notice there are not that many *USA* exit relays, If there
are no available exit relays in the *US*, is TOR smart enough to fail over to
the next available exit relays or must that be included in the `.torcc`as well

Yes, but not the Bridges you need to configure. Bridges are "entry points"/"safe gates" to enter the Tor network. You need to select an exit nodes/exits, and that's how it's done in torrc config:

StrictNodes 1
ExitNodes {us}

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