DURING AN ENG START, PRIOR TO THE FLT, THE L ENG BACKFIRED 2 OR 3 TIMES. A WITNESS DESCRIBED THE 1ST BACKFIRED AS 'THE LOUDEST I HAVE EVER HEARD.' WHEN THE ACFT TOOK OFF, TWO PERSONS LIVING NEAR THE DEPARTURE END OF THE RWY NOTED THAT THE ACFT (BELIEVED TO BE THE SAME) SOUNDED DIFFERENT & HAD A 'CLUTTERING SOUND. 'WHILE CLIMBING, THE PLT DECLARED AN EMERGENCY & REPORTED A FIRE IN THE L ENG. HE WAS VECTORED TOWARD ALTUS AFB WHILE DSCNDG IN IFR CONDITIONS. HE DSCNDD INTO VFR CONDITIONS & SAW THE ARPT ABOUT 3 MI AWAY. A WITNESS SAW SMOKE & FIRE TRAILING FROM THE VICINITY OF THE L ENG. HE STATED THAT THE ACFT'S NOSE CAME UP, THE PLANE ROLLED TO THE LEFT, THEN IT ENTERED A STEEP, NOSE DOWN, INVERTED ATTITUDE JUST BEFORE IT CRASHED & BURNED. AN EXAM REVEALED THE ENG EXHAUST MAINIFOLD HEADER ASSY HAD FAILED & ALLOWED HOT EXHAUST GASES TO ESCAPE IN THE ENG NACELLE IN THE VICINITY OF FUEL & OIL LINES. CORROSION HAD PROGRESSED THRU THE HEADER ASSY. AD 75-23-08 REQD GEN INSPN, BUT CONTAINED NOTE 'DO NOT REMOVE CLAMPS.' NO RECORD OF REPLACEMENT IN 6100 HRS