I was still worried about down-sizing to 3/8" fittings and if it results in a reduction in flow. So, did a partial flow test. The procedure is to check the flow by disconnect the cooler return tube and measuring the rate of flow in a graduated cylinder or something similar while running the engine at idle with the transmission in neutral.
Unlike the 4R100 mated to the 7.3, the 5R110 has a temperature valve in the pump so flow rate will vary as a function of temperature. These are the specs as per Ford:
44°C (112°F) 236.6 ml (8 oz) 15 sec
78°C (172°F) 473.2 ml (16 oz) 15 sec
82°C (180°F) and up 1893 ml (52 oz) 15 sec
My results were 10 oz. at 110F and 19 oz. at 176F, respectively. Didn't attempt the test at anything hotter. These results suggest that flow rate is comparable or even slightly increased with the low pressure drop 40k Tru-Cool cooler.