Thomas, that is the location I had in mind, although I would defer to the more experienced people on the board. I was looking at my 1985 300D last night to pick a location for the tee connection and was surprised at just how small these pressure lines are. I guess the small diameter doesn't hurt anything because all these lines do is sense pressure, so there is no pressure loss like there would be in a line carrying an actual flow rate. It would be interesting to see whether a gauge connected directly to the manifold would show greater pressure fluctuations than a gauge at the location shown in your photo.

I just ordered an Isspro gauge which I plan to connect at the location shown in your photo. I think I will mount the gauge in the ashtray.

I also think jba's point is well taken.