Other extensions (Tac Star, Choate, I believe) are made to the 18" length. Not as nice as Simon's extensions, but they work.
Are you sure you have a Police barrel on there now? I've owned a ton of 870's and I think all my actual Police barrels were 18.5". Normal ones floating around were all 18". Simon's extension (or the stock Remington for that matter) would be flush on an 18.5" barrel.
Could be wrong though - I'm sure they changed sizing conventions a few times over the years.
If you DO have a confirmed police barrel - keep it - especially if the choke is anything other than CYLINDER. Not worth swapping to the express barrel just for aesthetics.