You're going to have to man up and get a 1/2" breaker bar to tighten the nuts to get a good seal - if you're smart you'll put some antisieze on the studs first, and use new nuts.
You have to crush steel with two nuts over a dome to get a good seal, and crush them pretty good. When you think you've got everything bolted up and sealed up, leave the car in the air and fire it up. Move your hand around all the joints and flanges to feel for the exhaust coming out. If it's moving out, turn it off and get back under the car and tighten it up some more.
No, it isn't easy, and no, you won't be able to apply enough torque with a 3/8" ratchet. My 3/8" impact wrench didn't have the ass to tighten it properly, and I didn't want to risk snapping off the stud with my 1/2" impact.
Take your time, tighten slowly and smoothly, alternating between which nuts you're tightening to uniformly crush the steel on the X pipe to the dome on the exhaust manifold.
P0174 is for the driver's side. This can be caused by an exhaust leak or a damaged upstream O2 sensor. My money is on an exhaust leak at the manifold/x pipe connection.