John,
There are a whole bunch of oil return holes from head, through the pushrod area to the pan, so anything you use to douche the top end with will end up in the crankcase.
This fact doesn't make it impossible to clean the top end. I have used diesel fuel and a scrub brush to clean the area up. Pull the drain plug and put a pan under it. Scrub and clean to your hearts content.
When you are finished, dump a bunch of clean diesel over the rocker area just as a final flush for the stuff in the pan. Fill the crankcase with oil and change the filter if it has one and you should be on your way.
I have also done this on SBC's with hydraulic lifters, and it has caused no problems. The diesel acts as a mild solvent and has some lubricating qualities as well.
Stuart
