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Rating a cooler by vehicle weight always seemed odd to me. I'd imagine trans fluid temp varies with much more then just weight of the car it's in.
And the transmission has to work harder if you're loaded or towing. Wouldn't the 24000 GVW rating mean that it is capable of keeping the temps down when you're hauling 24000 lbs around? I know a Dart is a solid old car, but it surely doesn't need that level of overkill. Unless you were planning to drive a lot of uphill in desert conditions, like Bakersfield to Vegas for example. Then I'd recommend doubling everything-- radiator, tranny cooler, AC condensor, tire pressure... well no, maybe not tire pressure. 
I don't know exactly how they rate an auxiliary cooler based on using it in conjunction with a factory one, since not all factory coolers are created equal. Some tranny coolers (i.e. old Mercedes) are separate units that look just like little radiators, using air to cool. Other tranny coolers are embedded into the bottom of the radiator, which is mostly cooled by water (since even hot radiator water is cooler than tranny oil.)
Since you have a line on a cooler of a workable size, and you're planning on installing a temp gauge anyway, I'd see if this unit is capable of keeping the temps where you want them. (That 175-195 figure seems a tad low, if it's degrees F-- recall that most trans gauges are higher ranges than most radiator gauges.) If it ends up being not sufficient, then go back to the drawing board and redesign, and sell the tranny cooler to a guy in a Toyota who's driving to Vegas.
I wouldn't normally recommend designing by trial and error, except that in this case it strikes me you're probably not very far off.
- Erik
PS. Yes, it will work a LOT better if you install it so that air flows through easily. That GVW rating goes out the window if you install it sideways, or under the passenger's floor or whatever.