The gear venders seem really reliable and they have excellent customer service from what I have seen with Drag Week people. The 518 has a better overdrive ratio but I imagine that it costs more HP to run over a 727 and GV combo but I don't know for certain.

I keep kicking around overdrive with an A500 behind my small block but have never pulled the trigger. My motor loves causing at 3500 its super smooth and quiet, but when I do drag week I wouldn't mine being able to run 10mph faster lol. 65mph on some of the major interstates can be sketchy sometimes.