Jet and adjust the carb a bit and fix those oil leaks and exhaust leak, that's a killer. Make sure the frontend and brake parts are in good condition and buy some new shocks if it needs them. Then a big pair of sway bars. Then some dynamat on the floor.
It sounds to me like there are a bunch of mechanical loose ends on the car and spending a bunch of money on a new item and maybe not installing it with 100% care won't fix any ills. For example, the O2 sensor won't read the AF ratio correctly if that exhaust leak is ahead of it, which it most likely is.
Sheldon