Ok, let's start from the beginning.
The evaporator, wether cased or uncased, is the first part of the duct system.
It's not designed to bypass air around the coil. How are you ever to get any efficiency out of your air conditioner if your only running a partial amount of air though the coil? While there is leakage around the case/duct and coil, the vast majority of discharge air runs though the coil.
The one big item that I've asked several times in all your threads, where is the load sizing??? I've never in my entire career told someone they just need a bigger coil so more air can pass through.
What your getting yourself into is a mismatched nightmare.
You used automotive references in your other thread, so I'll stick to that-
I take my car to Johnny's 15 minute service shop for an oil change. Being the average car owner, I don't really know what oil it takes. They tell me they don't have any literature on what oil my car takes, and ask me what I want them to put in. I just point to the computer screen and tell them that one. Now, will it lubricate my engine? Yup, most likely. But the car isn't going to run anywhere near as long or efficient as if the proper oil was used. Hell, the engine might even blow up.
The same thing is happening here with your furnace. Some guesstimating is going on in the hopes you'll have an operating system in the end.
You should have a hard copy of your homes needs in terms of heat gain, loss, and the ducting requirements to move said energy.
Now, I saw in your other post your furnace is putting out a discharge temp of almost 170*, perhaps beyond that since you were using an inferred gun to measure the duct.
You need to figure out why your not moving air, or even if your ducts are anywhere near correct, or you will cause a quick death to new equipment.
