Can't help on the numbers, but if you pull a cylinder head you can check the engine bore, but use a caliper. 289 is a 4.0 inch bore. 260 is smaller but I can't remember by how much. Does the motor still have the original intake? I am not an expert, but don't recall the 289's before 65 coming in 2 bbl configuration, nor a 260 coming in 4 bbl but I could easily be wrong about that.
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