The original clearance seems to vary. I have a 2004 model and started with almost 3/4". It looks like the 2005 models (some, maybe just the 300hp version) have significantly less - probably due to the new plenum design.

 

I've got the 2005 and I have no where near enough clearance to add a spacer. I guess I need to go check out what they did when redesigning the plenum. Maybe I don't even need the spacer.