I'd say if you are going to buy them, get the 1.6:1. Another set of slow 1.5 rockers is a waste of money; might as well just use the stock ones. With your older cam design, the conservative ramp rates should have no problem with the faster rocker. Even the factory Magnum small blocks had 1.6:1 ratio, and really, 1.6 isn't much ratio either, compared to some engines like BBC or Cleveland, which are in the 1.7's stock.

Duration at .050 is measured at the tappet (tappet rise spec .050-inches), so that specification is independent of rocker ratio. Although the duration will remain the same, it will act like a slightly bigger cam in terms of idle and vacuum because of the increased valve overlap area.

Just make sure you have enough spring for coil bind clearance and have retainer to guide (or seal) clearance with the added lift.
-dulcich