There are a number of differences in the 1973-1987(91) trucks, the major difference is in the cowl area.
1973-80 trucks cowl area where the safety catch was is all enclosed. Wiper motors arm mount using 2 holes horizontally. Hinges for the hood mount on the fenders.
1981-1991 trucks cowl area has a different shaped area with a provision for the hood hinges, (springs mount to the fenders). Wipers arms mount with 3 holes instead of 2. Note the reinforce lip which is longer than the 73-80 version that the hinge mount bolts to.
1973 trucks NEVER had drip rails installed from the factory, they became optional sometime in 1974 and by 1975 they were standard equipment. 3 point belt provisions were offered but were not standard until around 1977.
In addition to the above, floors started to be galvanized in 1983 along with a plastic seal attached to the floor pans where most were known to leak.
1
