That sight won't work that good on a sidelock unless you want to drill and tap holes at the back of the barrel. Even then it won't work as good as one mounted on the tang. A peep needs to be mounted as close to your eye as possible.
TC made a peep that mounted on your tang. It's a #7194. pretty hard to find now, and when you do $100 is a good price.
Williams also makes a sight for your gun, but it's pretty bulky. I prefer the TC sight if i'm going to use a peep.
Personally, I gave up on peeps, and just use open sights with a 6 o'clock hold.