Nice bike.
I also have a 78.
Getting the air cleaner out is not hard. Two bolts on top of the air cleaner - just loosen them. The bottom half of the air cleaner housing can then drop down, then out to the left.
FYI. I've read (on this site) about the non-Honda air filters being sold on ebay (and other sources, too, I suppose) that are too restrictive, causing carb/fueling problems. I bought a K&N, and it works fine.