How to remove collectors / headers from a 911?

What tools are the correct tools to remove headers with heat exchangers from a 964? (angle grinder is very tempting at this point
)
So far I managed to get the four hex bolds lose with a T-handle hex 8mm tool. But the other ten 13mm bolts are impossible to get too.
It didn't work with a 13mm wrench, a 13mm socket fits but there is no room left for the wrench even with a flex extention or U-joint.
Whould a double shocket wrench be the correct tool?

So far I managed to get the four hex bolds lose with a T-handle hex 8mm tool. But the other ten 13mm bolts are impossible to get too.

It didn't work with a 13mm wrench, a 13mm socket fits but there is no room left for the wrench even with a flex extention or U-joint.
Whould a double shocket wrench be the correct tool?