No idea what's in the Chassis Manual says but from the factory the standard bar for 318 cars and heavy duty \6 cars was an .850" bar, 340 cars came with heavy duty suspension and .870 bars and a 318 car with heavey duty suspension would have had the .870 bars. I have a set of .850" bars that came out of my 68 Barracuda. My car was a \6 car with heavy duty suspension. It had the .850" bars up front and 6 leaf springs in the back.