couple things.
several decades ago I was doing a build, and it wouldn't boot no matter what I did. This was back in the days of DOS, and switches, and jumpers... no PnP
FINALLY ended up elmer's glueing a paper washer on top of each standoff before I mounted the board. Started right up no probs - maybe that's where your short is too??
also, I have a full set of waterblocks for a 780i That I'd be willing to let you BORROW.... or buy DS LMK
Try the washers - they are made out of heavy dute paper that looks the same as shipping envelopes. MicroCenter has them. Prolly an ofice supply too. OR, hope depot sells nylon washers that would work.