1) It will help if the flapper is a problem because you'll wire the flapper open when you convert to a pressure actuator.

2) Why don't you want to tee into the ALDA line for a boost gauge? That's where I get a boost gauge signal.

3) There is a grommet between the brake booster mounting boss the cabin heater coolant supply line under the wiper arm. The lines for engine shutoff, ACC vacuum, headrest actuators and some wires go through the grommet. There's room for more lines. Cut through a nib and feed your boost gauge line. I'm pretty sure you can fish out the line from below the dash but it's easier to remove the gauge cluster to fish out the line.

4) You can tap the same location for a pressure actuator signal. You can fabricate a cover plate for the EGR port with a nipple as you described. No need to fabricate a gasket if you don't mind buying another EGR gasket.

Sixto
87 300D