The trumpets are about 15mm off the plenum wall.

I modified a plenum cover to go on my Dad's Sapph when he went onto 750cc injectors with the MD240R and opened up the elbow and more than doubling it's width. I also had baffles fitted into it to stop the air being directly forced into No3 trumpet and partially into No2.

From this experience I would say that modding the elbow is a better course of action than a spacer but the way the elbow is modified needs some carefull thought..