E320 I6 manifold removal help pls

I am about to change the head gsk on this 1996 E320 I6 and have prepped the exhaust manifold for head removal but the manual states remove the cap screws in the inlet manifold and nothing else . I find my 6mm allen keys wont slid in as expected and access to the manifold screws on the I6 is quite difficult so can anyone offer advice on the right tool and methodology . Any other advice re the gsk replacement is appreciated
Tnx