Yes, your total duration will not change and is determined by the cam only. However, your duration at .050 is a LIFT measurement, and if LIFT changes from a different ratio rocker, your duration measurement at .050 LIFT will be different.
And 1.7's DO NOT work with all heads. Try running some on windsor jrs like I did and you will have problems unless you machine the pushrod openings larger so the pushrods won't rub on the castings.
Now wait a second. DURATION is a measure of rotation the lobe covers on the cam base circle (degrees) and LIFT is a measure of the difference of the lobe peak and the base circle radius (inches). Maybe that is what you are trying to say, but it came of slightly confusing.
Duration measured at 0.050" is a popular method of measuring lobe duration from 0.050" lift (opening) to 0.050" lift (closing). Sometimes you can find specifications for duration on cams measured 0.010"-0.010" and smaller, though this is less popular, as the smaller numbers are harder to measure and do not significantly add any insight on lobe design over the 0.050" method (you don't flow much air with less than 0.050" lift, do you?). Notice it is still a degree measurement.
Roller rockers (or any type of rocker, really) will multiply cam lobe lift by their ratio. If you have a cam that has an intake lobe lift of 0.278" (stock HO) and 1.6 rockers (again, stock HO), then your effective valve
lift is 0.278" * 1.6 = 0.4448"
. However, with the 1.7 rockers you will see 0.4726" valve lift.
The advantage of the 1.7 roller rockers (besides the reduction in valvetrain friction and wear) is that the desired lift can be achieved without using a cam that has more lobe lift, which results in steeper ramp rates and more aggressive grinds.
This being said, dyno reports I've seen indicate less than 10 HP gain from 1.7 RRs over the stock pieces on stock heads with stock cams. However, they may pay off in valvetrain stability, reliability, etc.
Sorry if some of this is redundant, I wanted to make sure none of us confused your original plans. I would say if you are going to buy RRs either way, go with the 1.7s.