Charge air goes the other way - from the turbo, up the EGR mixing pipe to the crossover pipe. A 1/4" drive u-joint should get you to the last bolt. Try a wobble tip hex key if the bolt head has an internal hex. Once the bolt is off the mixing pipe should slip off the turbo. There's a similar green o-ring on the turbo outlet flange over which the mixing pipe slips.

Are you lifting the exhaust manifold with the head? If not, are all 12 studs out of the way? If so, did you remove the turbo oil feed line that snakes through the exhaust manifold?

Sixto
87 300D