Crudely measured, the LD4B is right around an inch shorter than the Air Gap, measured with a strait edge from the high end carb pad to the top edge of the casting between the ports.

I ran a half inch wooden spacer with an AFB, the Moroso 3/4" drop base and a 2.5" element and the pro-flo top I could use a fancy wingnut without problems on that setup, but could not run a 3" element. With this setup, I can't run a fancy wingnut on the air cleaner.

If you'd get a deeper drop on the base and ditch the spacer as you say, I'd bet you'd be good with a 3" under the Rallye hood. You might even be able to get a thinner spacer in there.