[Grahame Hutchison]

Nigel, the Modifications section of the FAA Registration for N224HR lists an additional 298 US Gallons, to supplement the standard 648 US Gallons. A rough calculation would suggest that the range would inclease to around 3,401 miles (based on the SeshAir Inc Merlin III Specifications).

From FAA Registrations:

INSTALLED TEMPORARY FUEL SYSTEM RESTING ON PLYWOOD SUPPORTING ON EXISTING AIRCRAFT STRUCTURE. PLYWOOD BASE IS INSTALLED TO FACILITATE EVEN LOAD DISTRIBUTION. AIRCRAFT FLOOR LOADING LIMITS ARE NOT EXCEED ED FERRY SYSTEM CONSISTS OF 2 RUBBER AIRCRAFT TURTLE PACKS TANK. THE TANKING SYSTEM TOTAL CAPACITY IS 298 U.S. GALLONS. TANKS ARE CONSTRUCTED OF DOUBLE OVERLAPPED NYLON, COATED. CONSTRUCTION OF TANKS MEETS THE STANDARDS OF U.S. COAST GUARD 33 C.F.R. 183.580 SPECIFICATIONS. TANKS ARE PRESSURE AND LEAK TESTED IN ACCORDANCE WITH FAR 23.965A(1) AT POINT OF MANUFACTURE; TURTLE-PAC PTY. LTD. AUSTRALIA. ALL TANKS ARE SECURED TO SEAT TRACKS WITH CARGO TIE DOWNS. THESE TIE DOWN STRAPS ARE OF IDENTICAL TYPE THAT HAVE BEEN UTILIZED IN PREVIOUSLY FAA APPROVED TANKING SYSTEM STRAPS, (2 EACH) SECURE THE TANKS LONGITUDINALLY AND LATERALLY. FERRY FUEL TANKS ARE ELECTRICALLY BONDED TO THE AIRFRAME TO FACILITATE STATIC DISCHARGE, AND ARE VENTED TO THE ATMOSPHERE, AS OUTLINED IN AC43-13-1B. CHAPER 8 SETION 2 PARAGRAPH 8-33. THE FERRY TANKING SYSTEM IS PLUMBED INTO THE AIRCRAFT FUEL SYSTEM AS NOTED ON THE AIRCRAFT FUEL SYSTEM SCHEMATIC. THIS IS CO-LOCATED WITH, AND IS A PART OF THE FERRY FUEL MANAGEMENT DETAILS LOCATED IN ATTACHMENT 1OF THE TANKING INSTALLATION PACKAGE. AIRCRAFT FERRY FUEL SYSTEM OPERATION AND MANAGEMENT DETAILS IS OUTLINED IN ATTACHMENT 2. CONTINUOUS AIRWORTHINESS COMPLIANCE NOT APPLICABLE DUE TO TEMPORARY INSTALLATION. INSTALLATION TEMPORARY WEIGHT AND BALANCE IS COMPUTED ON THE ATTACHMENT TO THE FERRY TANK INSTALLATION FAA FORM 337 DATED 4/21/2014.

From Plane&Pilot:

STANDARD DATA: (III) Seats 8-11. Gross wt. 12,500. Empty wt. 7,400. Fuel capacity 648. Engines two 840-shp AiResearch turboprops.
PERFORMANCE: Top mph 325. Cruise mph 288. Stall mph 96. Initial climb rate 2,530. Range 2,860. Ceiling 28,900. Takeoff distance (50') 2,150. Landing distance (50') 1,570.

STANDARD DATA: (IIIB) Seats 8-11. Gross wt. 12,500. Empty wt. 7,800. Fuel capacity 648. Engines two 900-shp AiResearch turboprops.
PERFORMANCE: Top mph 355. Cruise mph 345. Stall mph 95. Initial climb rate 2,825. Ceiling 32,500. Range 2,790. Takeoff distance (50') 2,790. Landing distance (50') 2,874.

Some different range numbers here