Sometimes on these old trucks the outside of the crank pulley (the part with the timing marks) slips on the inside portion which is connected solidly to the crankshaft. The outer portion of the pulley floats on a rubber bushing (to damp vibrations), which can get old and allow slipping. If that happens your timing will look right but be wrong.

Best way to check this is to remove the number one spark plug, insert a wooden 1/4" dowel, and turn the engine until you have it at TDC. Then check the crank pulley timing mark to see if it matches.

Note: This has nothing to do with your timing belt. It only affects ignition timing, not valve timing. The fact that your distributor is rotated fully CCW to get 10degBTDC suggests that either your crank pulley timing mark is off (as per my suggestion above), or the distributor is inserted incorrectly.