Once that spacer is in, I don't know of any filters that are flat enough (or have a base that is sufficiently dished to basically drop down around the carb so that you can still have the engine screen). This also means that a round air filter may not allow you to close the decklid - my car came with a newer round filter that went partially under the decklid. I'm not sure if stock air filters were entirely under the engine screen, so don't slam the deck shut until you know you've got clearance.
Typical solutions:
1) Remove the screen and store it - and, if there isn't clearance for the deck lid to close, get either a smaller circumference round filter, or go for an oval filter.
2) Cut the screen to allow the filter to poke out (to allow going back to stock, you could try to track down a second screen to cut, or have a replacement fabricated with a hole, and store your original)
3) Stick with round and - I don't even want to say it - cut the deck lid so it fits (this is usually only done if you're putting in some major HP upgrades) ... e.g.
Simplest and probably most common solution is #1
Good luck!
BTW, I also think you should take a look at timing as a significant part of the equation, you may find with tweaking that and a few other items folks have mentioned, the spacer won't be needed at all.
