The main bolt holes in the later engines (built after 1959) like C2AE are deeper than the early engines such as the ECZ
The ECZ engine have about 0.6875 " (11/16) bolt contact in the block. The bolt holes in the ECZ blocks are usually 1.1875 deep so can be bottom tapped deeper.
When I rebuild the y-blocks I use ARP studs and turn them in finger tight after I have tapped deeper the thread in the block. This method still gives the stud thread near twice the thread contact the ECZ block previously had from the factory when using the factory bolts.
John Mummert recommends drilling and tapping the block even deeper until near in the oil gallery like the C2AE blocks are. I don't have an end milling machine to do that type of work to drill and tap deeper the block than was originally end milled. The local machine shops are to busy with the big bucks oil patch business to tinker with an engine also.
Its to cold here to go measure the C2AE block I have in the shed.
marv