The Bonesteel and Northeast Arms rails have similar top covers intended to allow co-witnessing w/ iron sights. I prefer the slimmer profile of the NEA lower rail over the Bonesteel one, but if needing full rails, you should really go w/ the bonesteel ones as the rail mounting blocks vent holes on the NEA...
All others that I'm aware of do not allow this co-witnessing.
However, with forward mount scopes/red dots and the 7.62x39 round, I really doubt that any polymer instability will have any noticeable affect on your shooting precision.
The only major ding I've heard about the FAB is that if you enjoy rapid shooting the polymer can melt...
That said, if you're looking for a precision scope mount, I'd personally lean towards a more typical rear scope setup, the railed dustcover is probably the best option:
http://www.vz58rifle.com/products/duralovy-kryt-zaveru-sa-vz-58-dostupne-v-cervenci/With the receiver side mount scope attachment being the second best option.
Here's the cheapest metal rail that I've seen:
http://centerfiresystems.com/hdg-vz58.aspxThe BC Tactical Rail system also looks interesting and they have a scout scope mount that replaces your rear iron sight for a scope moutn (just don't let your scope cover your ejection port or you'll ruin it!).
http://www.bctactical.ca/products.html