DURING A LOCAL SIGHT SEEING FLT THE PLT ENTERED A RIVER VALLEY IN MOUNTAINOUS TERRAIN AT ABOUT 1,000 FT AGL. AFTER ABOUT 5 MILES, THE PLT DECIDED TO EXIT THE NARROWING VALLEY WITH A CLIMBING RIGHT TURN OVER A RIDGE LINE. ABOUT 2/3 OF THE WAY UP THE SLOPE, THE PLT REALIZED THAT THE ACFT MIGHT NOT MAKE THE TOP OF THE RIDGE LINE AND THERE WAS INSUFFICIENT ALTITUDE OR AIRSPEED TO ATTEMPT TO TURN AWAY FROM THE RAPIDLY RISING SLOPE SAFELY SO HE ELECTED TO MAKE A CONTROLLED LANDING AT NEAR STALL SPEED. THE ACFT CONTACTED THE MOUNTAINSIDE ABOUT 300 FT BELOW THE TOP OF THE RIDGE LINE. THE PLT IS AN ACTIVE DUTY MILITARY PLT ASSIGNED TO FLY LOCKHEED C-141B TRANSPORTS AND ALL HIS FLT TIME IS IN HIGH PERFORMANCE JETS EXCEPT FOR ABOUT 32 HOURS OF GENERAL AVIATION FLT EXPERIENCE OBTAINED 2 YEARS PRIOR TO THE ACCIDENT. THE PLT SAID THAT HE OVERESTIMATED THE PERFORMANCE OF THE ACFT AND HAD NO PRIOR EXPERIENCE IN SINGLE ENGINE PROP MOUNTAIN FLYING.