Take the 55 gallon drum lid, and cut a whole in the center smaller than the exhaust pipe.

Weld a flange around this hole to fit the inside of the exhaust pipe.

Rinse the smaller barrel out with soap and water, and run a vacuum to swap out the air inside with fresh air.  Volatile chemicals explode when mixed with air in the right ratio.  We are trapping the volatiles in the soap and water mix, and then taking the air ratio below the ignition point by doing this.  Leave the vacuum running while doing the cut to be safer.

Put a wrap of flat tape around the barrel to get a cylindrical mark, then cut along the edge of the tape. Try to be straight to avoid a lot of extra welding later.. It should be 1/3rd as tall now.

Cut a hole in the center of the smaller barrel part to match the outer exhaust pipe diameter.  I left the vacuum running while doing this.

Slide the smaller barrel over the exhaust pipe, and put it on the larger barrel lid.  Mark around the edges.  Remove it.

Cut the air inlet pipe roughly in half, at a 30 degree angle from it's length.

Inside the circle you just scribed on the 55 gallon lid, mark off two ovals as far out as you can and still be inside the circle.  Cut them out.  Keep in mind your air inlets are angled quite steeply at 30 degrees off horizontal where they attach to the lid, so your holes will end up being long ovals.  Use the cut off air inlet pipes to mark them.

Add the air inlet pipes to the lid, and tack weld them in place.  Fit the lid back on the barrel to make sure they don't extend too far out as to hit the inside of the barrel when closed.

Finish welding around the air inlets.  An airtight weld is not needed, but it does need to withstand some wide temperature fluctuations so some thick spots help.

Cut the fan air inlet tube off near the end, at a steep angle.. again about 30 degrees off from the length.   Place the cut part against the side of your upper barrel and mark it.

Cut out the hole.  Make sure you are rotating the air in the same direction as your air inlets.

 <EDIT> I suppose you should really determine where the inlet needs to go first to line up with the motor, but that would entail describing the build of the fan housing first... which is not something that is going to fit *here*.  I need to re-think this...</EDIT>

Put the fan air inlet in the hole you just cut and tack weld it in place.