I'm not too concerned about height and have always run some sort if spacer, but there's only so much volume to be gained by adding a spacer without it being 10 inches tall. Lol! The spacers are definitely a good tuning tool, but there's only so far you can go with 'em. In doing some research and looking at the pics of the manifold, the plenum does look to be real small. I read somewhere on the internet (so it must be true) that the plenum volume, optimally, should be around the displacement of the engine.....it doesn't look to be anywhere near 517".....maybe half that. Of course, there are a lot of guys,a lot smarter than me on here and hopefully they'll chime in on this!