This is not part of Endeavor's 305 S. Congress project, but it is part of the South Central Waterfront vision. It's a presentation made to the December meeting of the South Central Waterfront Advisory Board and is for the redevelopment of the parking lot in front of One Texas Center. There are three options for residential redevelopment including one with a 170' tower.
http://www.austintexas.gov/edims/document.cfm?id=332914
Here are the three proposals:
Here's the tower proposal: