You can scale the whole drawing with SCALE command using "Reference" option of SCALE command.
Start SCALE command, select all objects you want to scale, use one of the endpoints of the horizontal line you mentioned as Base point, than choose "Reference" option, than when AutoCAD will ask for the reference length - pick Base point again, than pick on the other endpoint of existing line, than type new length (16'3 in your case) and hit Return.

Specify reference length <412'-9">: Specify second point:
Specify new length or [Points] <412'-9">: 16'-3"

Maxim