If you disable EGR, you can use the port for an EGT probe without drilling. It's a central location in 602 and 603 manifolds. It looks like it's also central in a 617 manifold if doesn't mix with the #1 runner but it might be considerably cooler at the EGR flange unless you can get a probe all the way to the turbo flange.

Here's Jeremy's work on a 603.960 - Ashtray gauges for the W124

Sixto
87 300D