Playing with the ex duration on a build like that is really more like fine tuning.
Don’t worry about it.
Besides, the Stealth exhaust ports aren’t fantastic.
I’d use 1-7/8” or 2” headers.
Properly recutting the seats and blending the bowls on Stealth heads gets them into the 280 range.
1.6 rockers will likely require modifying the intake pushrod tunnels.