DISPATCHED WITH THE LNDG GEAR EXTENDED FOR MAINT REASONS; B737-300 DECLARED FUEL EMER WHEN DELAYS DUE TO THE RESTR PERFORMANCE CAPABILITY OF THEIR ACFT RESULT IN FUEL BURN IN EXCESS OF THAT PLANNED.

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WE HAD AN UNSCHEDULED PLANE CHANGE IN ORDER TO FLY ANOTHER ACFT TO ZZZ AFTER THAT ACFT HAD THE LNDG GEAR ACTUATOR REPLACED. THE FLT WAS TO BE FLOWN 'LNDG GEAR DOWN' IN ACCORDANCE WITH MEL. FLT PLANNING WAS COORDINATED WITH DISPATCH IN REGARDS TO FUEL REQUIREMENTS FOR THE NON-NORMAL PROFILE AND POTENTIAL ATC CONFLICTS IN THE FLT PLAN'S AREA. CLIMB OUT TO CRUISE FLT WAS EXPEDITIOUS. ENRTE FLT PLAN MONITORING CONFIRMED AN ON PROFILE FUEL BURN AT CHK POINTS. DUE TO THE MISMATCH IN ACFT PERFORMANCE INBOUND TO ZZZ; CENTER ASSIGNED THE XXX ARR VICE THE XXX2 ARR. A REQUEST TO REMAIN AT CRUISE ALTITUDE FOR AS LONG AS POSSIBLE WAS DENIED DUE TO CONFLICTING TFC WHICH INCREASED FUEL BURN SIGNIFICANTLY ABOVE THE PLANNED FUEL BURN. CENTER THEN ASSIGNED AN ALTITUDE RESTR OF TEN MILES WEST OF INTXN AT 10000 FT. THE ADDED RESTRICTIVE PROFILE RESULTED IN A SITUATION THAT WOULD NOT ALLOW SUFFICIENT FUEL UPON LNDG; SO EMERGENCY FUEL WAS DECLARED. THE FLT PROCEEDED TO LAND UNEVENTFULLY IN ZZZ WITH 4200 LBS OF FUEL. PRIOR COORDINATION WITH DISPATCH AND REVIEW OF THE FUEL REQUIREMENTS AND PROFILE WERE THOROUGH. EVEN THOUGH 3800 LBS OF CARGO HAD BEEN REMOVED IN ORDER TO MEET CLB LIMITS; I BELIEVE A GREATER FUEL MARGIN WOULD HAVE BEEN IN ORDER. THE GREAT UNKNOWN IS HOW A NON-STANDARD PROFILE ACFT WILL INTEGRATE WITH OTHER TFC IN CONGESTED AIRSPACE. A BETTER COURSE OF ACTION WOULD HAVE BEEN TO PLAN FOR THE WORST CASE SCENARIO AND ADJUST ZERO FUEL WT ACCORDINGLY.