As Ted mentioned, missing mains are a really big problem, as it takes alot of caps and time to get them close enough to fit tight and get line bored. The have to be really close to fit the block tight, not to mention that the thrust bearing has to be perfect.My last C2AE block I used had thick bores(not the norm for a C2AE) but was missing mains. I had a bunch of caps and it took a ton of time($$$$) to find a set that would work. They all ended up coming off the same doner block that had a core shift in the bores.
It came out to be a nice block but my machine shop bill was out of this world!
Save them as you never know and these block are becoming imposible to find as good cores, but I will always choose one with the right mains from now on.