Probably. The safe thing to do -- when in doubt -- is to find a truck like yours in the junkard and test fit it before you buy it.
Note that you CAN put R-134a in an R-12 system. It just needs to be flushed out. ANY junkyard A/C should be (unless it has just been opened for the first time). And a JY compressor open to the atmosphere for an unknown time is a big risk (get a reman).
The cost of flushing may exceed the cost of new equipment. About half the cost is in the compressor, and much of the remainder is the condenser. Those are the riskiest junkyard parts, as well.