So they are bucked to about 20 plus inches and 5' in diameter. If so I'd just go right after those with my full chisel chains, making lots of noodles. Standing them up on their ends might be a PITA without help either mechanical or human. Wedge them in place with some smaller rounds and go at it. Have some wood or plastic wedges around as sometime I've had big rounds try to grab the bar while halving them. If one decides to go over on you while you are cutting, get out of the way. Yea, pictures would be great.