EDS is not particularly problematic but it doesn't have modern OBD so diagnosing a no boost situation can be tedious. Since you're the fellow in Seattle, you're lucky to know gsxr who has particular expertise in 602s as well as 603s. Heck, if you've figured out 722.6 electronics, 602 electronics will be cake. But note, the suggestion isn't as much a customization as reverting boost control to what came in the 87 190D 2.5 turbo and early Euro spec 250D/TD turbos.

As for the mounts, I'd look into using the arms from a 126 with 603 since 126s use the same rubber mount/separate damper system as the 123. The 124 uses fluid filled mounts. Might be the same amount of work either way but I'd spend some time exploring the 126 arms hypothesis.

Sixto
87 300D