'98 FLD120 w/ 3406E LOW BOOST!

I have a driver in my above mentioned truck that I was looking to uprate the torque in. He said yesterday that it was only making about 25 lbs of boost and today it got worse...down to 20. This engine is a 6TS and its rated at 475/1650. When I drove this truck it pulled good at 34 lbs of boost, but the last 2 days something has happened. He took it to our local mech guy n he said the head gasket is shot, but there is no coolant in the oil, no oil in the coolant and everything fluid wise is normal. Can a blown head gasket leak cylinder pressure only or is he seriously mistaken? I had this engine inframed almost 2 years ago n no warranty left. Could it possibly be the exhaust manifold gasket instead??

 

Yea...I told em to check all that stuff out first n see what he could find...but I havnt heard back yet...

 

Well apparently my driver is somewhat mechanically uninclined haha. What he thought the mech said was a head gasket was actually a turbo gasket....WHEW!!