If it were practical (perhaps using large, high power lasers from orbit, to bore just barely non-level tunnels) I’d like to see the diversion take place from the eastern watershed (basically, high areas of the Missouri River basin) to the western watershed, iow, feed the Colorado with all of its hydroelectric dams and reservoirs with the annual flood.
No need for lasers. tunnel boring equipment is very fast and good. tunnel building costs might be made up for by lower pumping costs until you come to the foot of south pass.
However, once the water was pumped over south pass —the falling water in the pipes could be used to turn turbines to run generators that power the pumps that pump the water up on eastern side.
That said, there’s not all that much water available at the headwaters of the missouri (that is not enough to lower the missippi) in western montana and you don’t want to go as far west as Cody because there’s several ranges there between the missouri and the plains of wyoming.