cam duration

Comp is right. You would need around 230/236 duration to keep around 7.5 dynamic (the "magic" number) that iron heads can run on 91 pump gas with. You might be able to go one size down. But that's probably max. Anything smaller and you most likely will have issues.

Why don't you wanna run a 230/236 sized cam? Why do you wanna run smaller?