[A26B]

Couple of questions please.

1. Is the photo of the gasket as it is attached to the plenum, or did it come loose & was just laid back over, roughly where it is shown?

2. What brand and product did you use to attach the gasket?

3. You mentioned the mating surfaces "seem" to have good planarity.
A. Were you able to actually use a precision straight edge that would cover the entire surface, end to end?
B. Was the plenum AND injector housing checked for flatness, end to end, across the entire surface? Results? usually checked w/feeler gauge and locations mapped.)

Difficult to tell for certain from the photo resolution, but it looks like the gasket has good compression from L to R in the photo, and on the R end bolt hole. However, I do not see the same indications of gasket compression on the #1 cylinder ports. This is usually an indicator of mating surface irregularity.

The gasket also appears to have been affected chemically, by the buckling and distortion around the #1 cylinder ports.

If the gasket is laying in the same position as before the plenum was removed, then the outboard being "sucked in" by vacuum doesn't correlate to the position of the gasket on the inboard side.

Going to a thicker material with greater compressibility is treating the symptom, not the cause. Looking at the port work done to your plenum & injector housings, there is sufficient gasket surface present to retain the gasket, if the surfaces are flat and the gasket is properly installed.

If all you have to check flatness is a long steel ruler, check the flatness using both edges of the ruler to find out if the ruler is straight.

I would really like to resolve your problem. It's not the gasket and it's not your porting being too large, as I can tell from the photo. Something else is in play.