That "Test Pipe" might be ok, but seem like a lot of time and fabrication. there are parts already available with some scrounging around at the PNP`s.
Find a 78 or 79 W116 300SD, these are pre EGR. just remove the manifolds, and they will bolt right on the 85 617.952 engine. look real clean and not block off plates to make to plug off the EGR holes.
will have to replace the down pipe for a FED pipe, the Cali pipe sits up higher. might be able to cut out a section, shorten and weld.
Iam not sure on the Cali Turbo, might work if it is clocked around some.
the Cali upper oil pan doesn`t have the oil drain from the air cleaner. that would have to be worked through. the Cali set up is kind of messy the way it works.
May be able to use the Cali air filter set up with some modifying getting the air to the turbo. at least you won`t have broken air filter mounts and bracket to contend with. the filter does cost a little more, maybe this is why I have seen a few at PNP w/o a filter in the can
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Charlie