People get "steam cleaning" confused on many levels. There is a difference between high pressure washing with hot water and steam cleaning. Steam is produced after 312 degrees and always should be done at 500 PSI and lower. (actual steam can not be trapped and is very dangerous at high pressure), on the other hand most high pressure washers can do pressures up to 5000 psi with water up to 250 degrees. (I sell a full line of cold and hot pressure washers and steam cleaners at my job).
Now to clean your engine, since it is out of the truck. Use old spark plugs to cover up the spark plug holes, best to make a plate/cover of some kind to cover the head area and intake area. Try to concentrate the majority of the blast to the areas where you want cleaned. I would use about 2000 psi, this should be enough with a good grade cleaner and you should be able to remove everything you need to paint it with minimum amount of clean up after wards.