What are the stock valve sizes on a 92 5.0???...are they 1.90/1.60?

 

no...1.78 and 1.45 I think.

 

the 1.9s and 1.6s are on gt-40 heads I believe. gt-40s were put in cobras i think as stock.

 

correct. oh and the gt40's are 1.85 / 1.54 by the way - the gt40p's have 1.46 exhaust

 

oh ok, I didn't know for sure thanks though.. whats the most lift and duration you can have with those valves.. Im sure more than .540 and 270 duration correct?

 

That sounds about right, but in all honesty most anything over .500" lift needs to be checked for clearance anyways as a safety. I can tell you that anything over an F cam type specs most certainly will need flycutting - when in doubt - check manually - what may be ok on one engine may be different than on yours, since wear and tear will change your effective real lift.