This does not look like my trans per the images. To the best of my knowledge I have a TH350, with the governor on the DRIVER side. Certain of this as I had to use a governor calibration kit to adjust shift points after the gear swap.
Sure, the governor is on the driver side, but the round accumulator cap on the passenger side should look like yours too. The 200C and 200-4R, I believe, are the only "backwards" flow transmissions GM has with the pressure port on top and the return line on the bottom.
As far as the heat exchanger goes, it kinda doesn't make much difference which line goes where, although generally the return line comes out of the top line, although not always. The radiator is a cross-flow design on a G-body which means the coolant should be about the same temperature coming from the cross tubes entering into the tank whether top or bottom. But, since the coolant flow is down, it makes sense to run the transmission fluid upward against the coolant flow to maximize heat transfer between the two fluids.
As far as mounting an aux cooler, you would disconnect the return line to the transmission from the radiator. Connect one port of the aux cooler to the radiator port, and the other connection to the line going back to the transmission. You're effectively putting that aux cooler in series on the return line from the radiator to transmission. You're done. Theoretically, most aux coolers don't care which way fluid passes through them as it's a cross-flow air to fluid transfer anyway.