We can all fine tune our knowledge with these testimonies. Willy has a 3 hole 75 block that the 1st gen mounts will not fit. I have seen these blocks too. So there are some out there. That's why there is a kit. Aaron has proven the theory wrong that it's all the three hole blocks that are the ones that the mounts do not fit. It makes it a little trickier in determining which ones need the adaptation. I believe it has something to do with the application and/or year whether the 3 hole blocks fit. One application that I remember that doesn't work without adapters is a 400 out of a 75 Grand Prix. The only post 1969 blocks that are going to accomodate the mounts without checking are the five hole mounts. One thing that is a given is that not all later blocks are a direct bolt in.