My experience with aftermarket transmission coolers is on trucks but they typically mount directly to the A/C condenser via some heavy duty plastic "ZipTie" mounts that are run through the fins of the A/C condenser and then fastened on the other side by slipping the plastic fastener over the ziptie and cutting off the excess.
One of the vendors must have a trans cooler designed specifically for your mustang.
I don't think that 175º is that bad. Hell, it's being "cooled" by a, potentially, 185º "cooler"!
Automatic transmissions are designed to operate right at about the engine operating temperature or, ideally, slightly below. You don't really need to be concerned until the transmission temperature exceeds 200º F on a regular basis. As a matter of fact, it's highly recommended that you run the transmission fluid through the radiator "heater/cooler" as well as the aftermarket cooler. I've even seen transmission coolers that have a thermostat built-in to them that doesn't allow the trans fluid to flow through the external cooler until the transmission temperature reaches 180º.
With that said, I think that they are excellent insurance against overheating your transmission, and I highly recommended them for most automatic transmission equipped vehicles that don't already have some form of external cooling.
Unless you plan on towing a boat, or something, with your Mustang, you probably don't need a very big cooler, so get one that is appropriately sized for your intended use of the car.