WHILE ON A X-COUNTRY FLT, THE ENG LOST POWER BEFORE THE ACFT ARRIVED AT THE DESTINATION ARPT. THE PLT SWITCHED FUEL TANKS, TURNED ON THE BOOST PUMP & RESTARTED THE ENG, BUT APRX 1 MIN LATER POWER WAS LOST AGAIN. THERE WAS INSUFFICIENT ALTITUDE TO GLIDE TO THE ARPT, SO THE PLT ELECTED TO LAND IN A SWAMP & AVOID CRASHING IN A WOODED AREA. DURING THE LANDING, THE ACFT WAS BADLY DAMAGED. AN INVESTIGATION REVEALED THE PLT HAD CALCULATED HIS FUEL AT 65% POWER. ACCORING TO THESE CALCULATIONS, THE AIRCRAFT SHOWULD HAVE FLOWN 5 1/2 HRS; HOWEVER, ACCORDING TO THE TACH TIME, THE ENG LOST POWER AFTER 4.68 HRS. THE OWNER STATED THAT THIS PARTICULAR ACFT USUALLY BURNED APRX 10 GAL/HR. WITH 48 GAL OF USABLE FUEL. THIS WOULD HAVE YIELDED 4.8 HRS OF FLT TIME, NOT ACCOUNTING FOR START, TAXI, RUN-UP, TAKEOFF & CLIMB. REPORTEDLY, THE ACFT HAD CARBURETOR PROBLEMS PRIOR TO THE ACCIDENT, BUT THE OWNER STATED THAT THE PROBLEM INVOLVED THE ENG RUNNING LEAN AT LOW RPM. THE PLT DID NOT MENTION ANY FUEL GAUGE PROBLEM IN HIS STATEMENT.