Jack,
You have the right idea but I would call Griffin and find out if you have a "stock-type" trans cooler or a true heat-exchanger. With a true heat-exchanger, under extreme racing conditions you might see a 5-10 deg increase in coolant temps. Under those conditions I run 190-200 on water and 230 on
oil.
The nice thing about it is its simplicity, you don't need a thermostat because it won't try to cool the
oil below the coolant temp. When the water is warmer than the
oil, it will be heating the oil. It won't clog since the "dirty oil" had to pass thru .002-.003 bearing clearances too. As said above, pressure is not a problem.
Your idea is sound and widely used. Really the only question is what do you actually have. If you know it to be a "stock-type" coiled tube or something similar, it's pretty much worthless as a heat exchanger.
Scott