You can download the 64-bit version directly from Autodesk's servers: http://download.autodesk.com/us/support/files/autocad_2016_service_pack_1/AutoCAD_2016_SP1_64bit.exe .
Understanding Autodesk AutoCAD 2016 (64-Bit): Architecture, Features, and Legacy Autodesk AutoCAD 2016 -64 Bit- -Henvion-