I agree. You usually see stand off with stack injectors or individual runner carb stuff and not with a plenum.
That said, I agree the 2 inch spacers should help with any stand off as the plenum dampens those reversion pulses.
I can honestly say I have no idea what happens with reversion and stand off if the plenum volume gets too big. I don't even know that you can get too much plenum volume.
At this point, it's kind of spit balling.
Obviously, if you wanted to eliminate the spacers as the issue for the stand off you could swap the eddies on there and see if it goes away.