Is there really a difference..??

If your using the 318 heads use the LD4B. Much better fit . Take a straight edge and spit the intake runners up the middle and continue the center of the port to the top of the intake so it can be seen when laying on the engine.

After installing the intake bolts loose. Then align these marks with the center of each valve cover bolt hole in the head which is the center of the head port.

With the china wall pins installed once you have the intake at its best alignment tap down on the ends of the intake.

This will put marks where the intake is at for best alignment. Drill the intake for these pins . This will hold the intake where it will be fitted and work the best.

Why they don't come drilled? Is not that they were missed .

Many remove the pins or drill big holes we don't do that here. Just an FYI use it or not I thought I would mention what I have been doing since the 70's. If the holes were drilled for a different engine have them filled either with weld or a press fit slug of aluminum.