I think your talking about the Chevy 8.1L Workhorse. I used the Prestone flushing kit ($5 or so) It has a Tee that splices into the heater hose, and is much easer that removing the thermosat and disconecting the upper radiator hose.(unless your planing on putting a new thermostate). Be sure to turn your heater on so that the heater core gets flushed as well. Flush until clean water flows out. Open the pet cock valve on the radiator as well, so the radiator drains, it should be on the lower driver side of the radiator. I flush with the engine runing untill it gets to normal operating temp so that the thermostate opens up, and then conitinue flushing with the engine off to cool in down.
Before putting in new antifreez, let the engine cool. There are drain plugs on each side of the block, you need to remove those and let the block completly drain. I put a dap of high temp RTV sealent on the threads prior to reinstalling the drain plugs.
You'll need 3 gal of antifreez and 3 gal of distilled water if your going with a 50/50 mixture. Up to you if you want to use GM's Dex-Cool.($15+ per gal) I like the color green antifreez myself.