It's making a loud exhaust leak noise, smell of fumes, and a severe loss of power,

gas smell is probably from injector --hole in pipe usually causes loud noise and possible smell of exhaust in inside as to effects on driving and engine usually doesnt except if by o2 or cat can cause code

I am not sure what fumes you are talking about,but if they are exhaust fumes you smell there is probably an exhaust pipe leak,joint leak, or manifold leak. Take it to a reputable exhaust shop or garage for free inspection.

In most cases fumes coming into the cabin thru the vents is from a leak near the engine. If the exhaust is leaking around the engine it should be fairly loud.

vechicle could be burning oil.or look under hood for valve cover gasket leaks. oil leaking on hot engine and exhaust manifold will cause burn smell.check drive belt could be slipping.make sure brakes not dragging or if you replace brakes pads you will smell burn smell until new brake shoes or pads break in.

Exhaust manifold leak? That's not common. You talking about the VVT near the manifold at the top of engine above the power steering pump? Yes indeed, AC Delco is the only one our engines love. 41-103. Obviously you have P0304, what other codes?

A leaking exhaust manifold could make a ticking noise. So could a lifter or valve. The exhaust noise would be louder under load.
If the carb is running too lean, or you have a vac leak, the lean fuel mixture could make the exhaust leak worse, and cause the loss of power.

first place to check is the cat converter

It sounds as if you might have a vacuum leak (fumes and running cold) and an exhaust leak ( loud)

sounds exsaust leak some place close to air intake for your heater! is it loud?

Start the car open the hood. with the hood open listen and smell for the leak. Probably the frt exhaust manifold leaking. Independent mechanic can repair just as easy. No need for dealer. If the leak is bad enough you will smell exhause leak. Look for the area with the greatest amount of noise.