With newer flat tappet cams they already have aggressive ramp rates for the material and the design of the lobe to lifter geometry. Using a 1.6 rocker increases this ramp rate, meaning that it will cause the valve to open farther and then close in the same amount of time. This increases stress on the cam lobes and lifters. If you are just talking about using a roller rocker on a flat tappet set up, you can do it. In this case since you didn't spend a bunch of money on the rest of the valvetrain I don't see a big need to spend a bunch of money on the rockers.
As for the cam you are talking about, if the cam card says 2500-whatever, this is just a broad-generalized guesstimate made by the manufacturer. You also have to understand that these guesstimations are made based on an engine of 350 cubic inches. You are planning on a 383, those extra 33 cubes eat up some of the range on the cam (the larger the displacement, the more duration is needed to produce the same effect). So as a general rule of thumb you can take about 500rpm off the guesstimated ranges. If the cam card says 2500-6000 then it will be closer to 2000-5500.
I didn't look into the cam specs of the cam you are looking at but I have built and driven a good few 383s, and for different uses. What I have figured out with the experience is that street 383s like around 222-232 degrees of duration at .050 (keeping in mind 9.5:1CR, mild heads, dual plane intake)