Eager to see a valid answer to this one !
75 M docs... wow.. that's a lot of data right there.
I'll try thinking some basics facts: you should have no more than 1000 docs per node. So you will need 75.000 nodes.
And then you will need 75 super nodes, containing 1000 nodes each one...
Having said that, we will need more info about your 75M documents to state a solution: what type of attachments, size of e/o, do you need to index also the attachments ? what's the logic about your documents, is there any numbered sequence, alphabetical order, etc. etc.