Here is an illustration from the Master Parts Manual that shows the distributor plate.
Here is the the Shop Manual extact giving the Chevrolet instructions on replacing the tube that runs through the block. As Jerry said above, it is not real easy to replace; hence, the external line that is occasionally seen. I recall seeing an illustration in one of the shop manuals linked in the Old Manual Project, but I did not locate it in a quick perusal.

Hoyt