I never seen the tang on a t/c firestorm maybe you can post a picture of the tang,I have tried the williams set-up peep but for some strange reason I don't care for it I had it taped on a renegade for a test run before I would drill and taped it.
I like the t/c hunter peep for me it is an outstanding peep and with a fiber optic front sight opens a new door for old eyes.
I have the t/c peep on my PA, hunter hawken 50 cal. flinter ,you will have to open the peep hole to .090 for early and late evening hunts,the factory hole is .040 great for target shooting and sunny days in the field but not enough light for them last min: hunts.