In general any thing newer than 1980 could be metric. In a push to go metric, GM Started to convert body bolts to metric, first on cars then on trucks. Engine and accessory ( alt, starter, AC, PS ) would stay american, fenders, body bolts, etc would go metric. If a "new" transmisson was used it probably was metric. That is why some diff covers were 5/16 and others were 8mm.