Ever heard of a scrub brake? It was the way they built karts when I was a kid. A flat plate of steel on a pivot, almost laying against one tire. Push the brake, plate presses flat against tire tread, you replace worn out tire every week.
Today somebody's still using that method. Me. I need a parking brake that's not hydraulic, electric, anything like that. In other words, failsafe. I'll build it on with a parking lever that locks, and when my raised idle on that bad_ss engine tries to make it take off down the street when it cranks up, the failsafe is safe from failing.
If you can't get lockup from whatever brakes you end up with, even Oldsman's Mid S kit can't, then having that mechanical lever right beside you as not only a parking brake but an emergency brake as well might save your bacon one day. Especially if you have a sudden brake failure altogether.
Very cheap to make from scraps of steel, and really nice to have.
FORGOT TO ASK: Is that a live axle (straight-axle)? Hope so, that means both rears brake when you hit it. A scrub brake would do the same. I try to tell everybody that 1-wheel braking is absolutely unsafe and not enough. Even a scrub brake fails on a wet road, steel plates just skim across wet rubber. You try to lock up a single wheel, especially when wet, it's like Possum-boy said: Ain't nuthin' like that fine feelin' of flight when you bounce off that shiny Mustang bumper!