A 40G can have more than one CHE, although each one has to have it's own thermostat, or a thermostat that runs multiple devices.

It can also have a UTH. It all depends on how many thermostats you are willing to run.

If money is not an option, which in most cases, it is, I would have a UTH set to 89F for a hot spot, say on the left side. Then, I would have a CHE about the 1/3 of the way to the right of the tank, or a little more, to create an 82-84F ambient in the middle of the tank, and a 78F cool spot. OR, a small wattage CHE closer to the UTH and another closer to the cool side.

Of course, if money is not option, get a good PVC type enclosure, and you won't be having these issues.

Frankly, for a 40G, I think you are asking too much of one CHE. Sounds like it will be able to cool down for the hot spot, but then be too cool to keep ambient and the cool spot where they need to be, unless room temps are kept pretty high.

Not trying to be difficult, just my assessment. Happy to help you with a new configuration or with new tank suggestions, but I think it will be difficult to make the current setup work long-term and with weather changes, etc.