Shortly after initiating an enroute letdown, the acft experienced a catastrophic engine failure at about 4,500 ft MSL. Plt selected a road for an attempted emergency landing, but a car turned onto the road in front of him and forced a diversion to a nearby field. The acft encountered berms, ruts and clumps of bushes after touchdown. Examination of the engine revealed that the number 6 cylinder connecting rod had failed near the crankshaft end with high cycle fatigue evident over about 60% of the material thickness. Examination of the acft maintenance records disclosed that since manufacture in 1964, the acft had accrued a total of 1,299 hrs, with only 109 hrs in operation since 1976. Lycoming publications indicated that the rod had been superceded in production several times, with this particular part number not allowed for reuse at overhaul. The manufacturers recommended overhaul period is 1,800 hrs or 10 years time in service, whichever comes first.