I'd get it all and pile it up at the house. Free, I don't tarry on that price. I've had pieces like that before. They were all bowed down a little, that's just how they are. Most bridging I saw were cattle gaps over creeks going across farms. Oilfield uses them. Two on each side, eight feet long. We get them used. But for twenty one years that used cattle gap has had a loaded 18 wheeler crossing it every other day. One brings feed, another picks up product. If you go the entire 20 ft, weld two, one on top of the other, standing up like they are. I don't think you have to span 20 ft, that's more small river. You'll have more of a problem with it sinking in the ground, so put some concrete chunks under each end. Then lay some rebar on the blocks and pour dry sack Crete on that. Put a few buckets of water and call it good. Get it ready for a floor, but don't install floor. Keep all that weight off and you can maybe winch it across the creek. I've put in some bridges out of light poles, full 40 ft. Had to go back and fix it because the creek flooded and washed the bridge downstream a little. Put it back and cabled it to some trees on the up stream side. Stayed for years so far. It was just a neighborhood friendly project helping a bunch or old fellers. It's just we had log skidders, tractors, other equipment. The skidder winch did most of it. I don't know the tree situation around your creek.