Streetable 12 second 20+ mpg slant six?

I don't see much work involved to get a V8 20+ on highway, its 20mpg in the city that's the issue. Your jeep is also fuel injected and overdriven, unless it's a much older jeep. Not trying to fight ya, just saying the OP said he wanted a /6 for fuel mileage and the torque of a V8, while being streetable. Which is really the beauty of a turbo; you can have a smaller stock engine for light work and double the power when you need it.

To make a 12 second NA V8 car, you're going to need heads, compression, more CID, cam, exhaust, and the worst thing for the highway - low gears. For a turbo car it's basically a stock rotating assembly, small cam, lots of fuel available, exhaust and turbo plumbing, low compression, and high gearing.

Lots of V8 guys ask what LA head is better than the other to pick up a little port flow. Turbo does it all for you when you put a 16psi head on top of that valve, it doesn't really care that much. Not saying big ports, cam, and valves are a bad thing, but you're still waiting for the atmosphere to cram in the mixture.

In my opinion, the only time you're driving a turbo 6 at a stock NA power level is when you're sitting at a stoplight or maintaining speed, which is exactly what you want for mpg. You'll never really feel the stock power of a turbo 6 because as soon as you need more power from the engine, the turbo spools up and off you go.