THE PLT & 3 PAXS WERE RETURNING TO DALLAS, TX FROM A RETREAT IN MONTANA. THE PLT FILED AN IFR FLT PLAN FROM WHITE SULPHUR SPRINGS, MT, TO CASPER, WY, BUT DID NOT ACTIVATE IT AFTER DEPARTURE. THE ACFT WAS RPRTD OVERDUE BY FAMILY & A SEARCH WAS INITIATED. THE WRECKAGE WAS LOCATED BY THE CIVIL AIR PATROL ON 7/1/87. THE ACFT HAD CRASHED AT THE 10,000' LVL OF A BOX CANYON AREA IN MOUNTAINOUS TERRAIN. THE IMPACT AREA WAS ON A ROLLING MEADOW THAT WAS SURROUNDED ON THREE SIDES BY MOUNTAINS. ENTRANCE TO THE AREA WAS FROM THE NNE. THERE WAS EVIDENCE THE ACFT IMPACTED IN A LEFT WING LOW, NOSE LOW ATTITUDE, AFTER TURNING TO A HDG OF ABOUT 050 DEG. BEFORE TAKEOFF, THE PLT HAD ADDED 35 GAL OF FUEL (MIXTURE OF 80 & 100 OCTANE FUEL) FOR AN ESTIMATED FUEL LOAD OF 213 GAL. NO EVIDENCE WAS FOUND OF A PREIMPACT MECHANICAL FAILURE OR MALFUNCTION. APPROX 27 MI SSE OF CODY, WY, THE WX WAS IN PART: 18,000' SCATTERED, 25,000' OVERCAST, 30 MI VIS. THE ELEV AT CODY WAS ABOUT 5,000' LOWER THAN THE CRASH SITE.