The cam has to be specifcly ground for a 1.6 roller rocker. To get a 1.6 ratio compared to a 1.5 ratio you have to move the contact point where the rocker meets the cam.

When this happens it alters the cam timing. You could just have them put a V6 desing on the 4 cyl cam and that wll take care of the geometry problem. Or you could use the 2.0 roller rockers that have the 1.5 ratio.

If you use the 1.5 ratio you stll need a roller profile on the cam lobe to get the valve opening rates right. The roller has a much smaller radius and will open slower off the seat.

I have made measurments that back ths theory up and posted this on the starion board so people would understand what happens when you swap these rockers in.

You can get roller cams ground for the 1.5 rocker, though they tend to be performance cams woth more duration.
Ask about them on the starion board.

Kevin