rocker arm ratios

Yes, the 3.9 is a "Magnum" engine which came with 1.6 rockers like the 318 and 360 Magnums.

One must also be careful with higher ratios, especially when using OEM springs, because a valve opening and closing faster usually requires a stiffer spring to be able to keep up with it and not float. Unfortunately, most of us have don't have a way to figure out the minimum spring pressures we need to use when swapping around cams and/or ratios. So we have to rely on the cam company's advice. They are in the business to sell parts, but it is usually good insurance to go stiffer with higher ratios.