Maybe you got the cams off time and your pumping air at overlap and the O2 is displacing the burn exhaust.

You can see the bottom mark without removing anything BTW

 

i hate to be the first person to bring this up but you took off the head to do the headstuds right? did you check that you dont have an exhaust leak? cause if you have a tiny exhaust leak it wouldnt effect AFRs untill you were under boost, then it would suck air in caused by the vacuum of the air going out the exhaust, this pulls air into your exhaust upstream from your wideband and causes a lean reading AFR, chances are high you may have this problem since it was my issue for a long time

 

exh leak would suck at idle causing lean afr but I never seen it do that during load.

Check intake cam mark, could have skipped a tooth CW

 

Did you remember to plug the front 02 back in when you put the valve cover back on?

 

the vac line that comes off the turbo to boost control solenoid