MOUNT VERNON — Crews are installing a new section of the I-5 bridge over the Skagit River that collapsed four months ago and hope to have the crossing open to traffic Sunday morning.The state Transportation Department says the temporary crossing that’s currently in place will be closed at 7 p.m. on Saturday night. Crews plan to work overnight to slide the permanent section into place and be done within 12 hours.Travis Phelps, a DOT spokesman, said the bridge swap is a daunting task, requiring the removal of a temporary span that weighs 500 tons and the installation of a permanent span that is some 900 tons. Officials cautioned that the operation is complex and that the closure could be extended if things don’t go according to plan.“We’re confident, but we obviously want to do this right,” Phelps said.The overnight replacement is designed to minimize traffic impacts at the crossing that carries roughly 70,000 vehicles a day.Officials have been working toward this moment ever since an oversize truck load hit the bridge on May 23, sending one 160-foot section and two vehicles with three people into the water. No one was killed.Traffic was detoured for a month through Mount Vernon and Burlington until a temporary span was installed.The total cost of removing the temporary span and installing a permanent replacement is $8.5 million — paid by federal emergency relief funds. The contractual deadline for the permanent replacement span is the beginning of October.Along with the permanent span installation, the state is planning additional nighttime bridge closers in the coming weeks because crews also plan to retrofit the overhead bridge supports.