By itself, this isnt worth it, IMO. The breakdown:
1. going to 1.6 RRs - next to nothing, a small friction reduction, and yes, you will need your heads cut for screwin studs (if you have pressed- in now), guideplates, and hardened pushrods.
2. going to 1.7 RRs - the equivalent of going to a slightly hotter cam. Same work involved.
nothing wrong with RRs, I have them myself, but combine it with head porting, bigger valves, etc. Then your RR will be part of a good overall package, and you do all the work at once. I am surprised you would have screwin studs with rail rockers, that makes no sense, since Ford used rail rockers to AVOID having to use guideplates/hard pushrods/slotted pushrod holes. Are you sure they arent pressed in?
If your studs are really screw-in, then you have to check your geometry once you put in guideplates (raises the stud up), new pushrods, prob...all this is why Id do it as part of an overall head upgrade.