I've heard that at higher engine RPMs (like I get with a 4.86 3rd member), that the water pumps are moving the coolant so fast that it does not dwell in the radiator long enough to effectively transfer heat from the coolant. When you slow down, it slows the coolant flow and the radiator can do it's job.

Regarding Little Dearborn, I find them to be a bit expensive. I remember going in there in the 60's with my father to get a wiring harness for his 40 Ford 1-1/2 ton truck (jail bar).

I've found most of what I've needed, including my truck, on ebay. I've seen shrouds on ebay when someone is parting out a truck.