I'm just not a believer in flushing with the engine together. Seems every flush will go back for another cooler (or three!) or worse problems. With the engine in the truck, it is impossible to remove what the chemicals will dislodge..no matter how many times its flushed. The sediments that are freed by the chemicals were settled into areas of low flow, and hardened. Once broken free, they will be in the system for months. Unless you are able to run a 100% coolant filter (not a bypass), a flush is a bad idea.
Of course, this is just my opinion. Guys pay shops thousands every day for flushes. But I believe you are being sold the "idea" of clean, which makes you feel better, even though you are opening the door for problems. But if you can remove the engine and flush coolant passages while limiting the number of turns and connection/gasket points, you will instantly remove what is dislodged.
Hope I didn't hurt any feelings here, just my opinion. Sorry for the derail.