Help with placement of Aprilaire 700

Ok so I am a novice in the field of HVAC but am pretty handy when it comes to most things. My 2 year old son had an asthma attack so I am in the process of upgrading my air quality equipment. I purchased an Aprilaire 700 since it was one of the highest rated humidifiers and now that I have it, I just realized that the furnace in my basement doesn't look like the rest of the installations I've seen on youtube. See attached pics. I don't know if I can install this unit just above the coils and what would I use to cut the metal. Sorry if I sound a little confused but I don't know if I should install it on the supply or make it easy and install on the return. Please help..
Ok

 

In other words, Is that case above my furnace my supply line. I mostly see plenum above furnaces and not a metal housing. Just afraid to cut into that to install the unit. I know that Aprilaire says you can install into a return air duct but I would like the unit to operate at an optimum level.

 

I'd would of done the bypass unit since your space is very limited... But if you have to keep the powered unit, you can mount it on the side of the a/c coil housing if there space on the right side?

Remove the cover (Looks like theres TONS of putty or something on the seams!?) and you can see the coil, and know where to make the cut in the casing.

 

Thanks Jay for your help. I do have room on the right side of the coil housing and I can see that I have an A type coil. I am just wondering if installing this unit in the space I have be any better than a bypass unit. Since the coil housing is big I can put it up high. Aprilaire does say this unit can be installed on a return, near the coil, but most of the threads I read say this will not work that well. Don't know what I should do. Should I return it??????

 

You can mount it on the coil casing, I don't suggest setting it on the return since there is no heat to help with the water eveperation.

since you got the room, you can keep the 700.