To be honest I don't understand why you've even asked this question. You know your seating capacity, you know you don't want anybody to have to stand. There is absolutely no alternative but to cut people off the guest list... and the only way to do that is to suck it up and get started.

There's no magic workaround here - you've over-invited to such a large extent that you're never going to get enough declines to sort it out for you.