THE ACFT COCKPIT/CABIN FILLED WITH SMOKE AFTER TAKEOFF. THE PLT & PASSG SMELLED AN ODOR OF ELECTRICAL WIRING BURNING AND SAW FIRE AT THE FIREWALL NEAR THE RUDDER PEDALS. THE ACFT WAS TURNED 180 DEGREES AND LANDED ON RWY 35. THE 2 OCCUPANTS IMMEDIATELY EGRESSED. INVESTIGATION SHOWED THAT THE ENG LEFT EXHAUST STACK, PN 63726-O2, WHICH SHOULD BE INSERTED INTO THE MUFFLER ASSEM, PN 66704-00 AND SECURED WITH CLAMP ASSEM, PN 65442-03 WAS DISPLACED AWAY FROM THE END OF THE MUFFLER ALLOWING HOT GASES TO BE DIRECTED ONTO THE FIREWALL IN THE AREA OF THE STARTER SOLENOID AND BRAKE RESERVOIR. THE SECURING CLAMP ASSEM WAS PRESENT ON THE MUFFLER WITH THE NUT INSTALLED AND CLAMP FLUSH WITH THE OUTSIDE DIAMETER OF THE MUFFLER EXTENSION. THE LOCATER PIN WAS EXTENDING THRU THE MUFFLER EXTENSION. THE EXHAUST STACK HAD THE LOCATOR HOLD AND INSPECTION DID NOT SHOW THAT THE PIN WAS WORN OR SHEARED TO ALLOW DISENGAGEMENT. FIRE DAMAGED THE ACFT FROM THE FIREWALL TO THE BAGGAGE/REAR AERA. FLUID FROM THE MELTED BRAKE RESERVOIR APPEARED TO ADD TO THE FIRE.