At 10:45 mountain time, a Beech 58P on a post-maintenance test flight suffered an inflight fire in the left engine nacelle just after takeoff from Boise Air Terminal. Neither the pilot nor the mechanic was injured. The two were making a test flight after heavy maintenance and all preflight inspections and operations were normal. After the airplane climbed to about 20 feet, the pilot noticed flames coming from the left engine. He landed on the remaining runway. The fire damage was contained to the accessory section aft of the firewall, but one engine mount and the structure around the engine, including the fuel tank, were damaged.