I have a 1990 FLHS. I want to lower the engine speed at highway speeds
Factory final drive ratio is 3.37. It looks like in 94 Harley switched to a 3.15 ratio.
In my research the belt and pulleys are the same. Primary comp went from 24 tooth to 25 and clutch went from 37 tooth to a 36.
I'm thinking the comp sprocket is a easy swap.
My question is what's the easiest/cheapest way to change the clutch sprocket.

 

You're cheapest way out is a 65 tooth pulley and a 2 tooth shorter belt.. You will still have to take the primary down to change the belt..

 

Fellow 1990 FLHS owner here, owned from new. My bike is an International model, which came new with the gearing that 98 refers to above. Ironically I have changed my gearing to stock US spec, because I found it too high! If you do change, with a standard engine you may find you change gear more often. I now have an S&S 107 and still use similar gearing to your bike - tht way I get the benefit of the bigger engine, with more torque!

If you raise your gearing as suggested you will reduce torque at the rear wheel throughout the rev range.

 

I just swapped to the 65 tooth rear sprocket on my FXR and so far, I like it. It does drop the revs all around and for the way I ride, it's perfect. As a note, you can swap on the sprocket without changing the belt. The sprocket is smaller than your original one so will push the belt adjustment back farther. It's still within limits. No belt change required and if you're not happy with the gearing, just change it back. Easy.

 

I bumped my trans pully up to 34T (from 32) and my rear wheel pully down to 65T (from 70) cuz I'm planning a long road trip with lots of highway miles.

It's kinda like riding the 4 speed shovel with a 5th gear overdrive. 3000 rpm in 5th gear puts me at 73/4 mph. 3300/3400 rpm at 85 mph. She will purr right along at that speed all day long.

Since riding it with this change I'm thinking of changing from the EV13 cam to giving the EV27 a try.