What makes you think this is the case? There were far more hydraulic cam 273s produced than mechanical cam versions. All the 2 barrel 273s (which is what your car started with) had a hydraulic cam and in '64 the intake manifold bolts were vertical. If you wanted to put a 4 barrel carb and intake on a '64 273 it would be easier to swap 318 heads on it and get an aftermarket intake than it would be to find a '64-'65 4 barrel intake. If you had non adjustable rockers on a mechanical cam, you would have more noise than a shredder in a scrap yard.