Seth,
If the engine is in the car I prefer using the brush method on the cast iron surfaces. You can clean the entire engine compartment... frame, firewall, and engine areas with GOJO creme ( NOn pumice) hand cleaner. it's amazing how it will clean up the grease, oil, grime, is easy on the paint and does an excellent job. if you need to use a soft paint brush to get the nooks and cranny's. wipe it down good... a little water and a towel on the engine to wipe away any residue stuff and you're ready to paint. If your head nuts are plated you can use masking tape around them to tape them off... and use a foam brush or smaller 1-inch paint brush on the engine areas. trying to spray paint is a mess and hazard to everything surrounding and not as controllable as the brush method at least for in the car. You can also use this for the frame area, crossmember etc to clean things up. If you want to replace the head nuts you can do them one at a time..... just know there's risk in doing that. Stud breakage...retorque, etc. If the manifolds need surfaced or other service done it's the perfect time./ Also removing the starter and generator on makes for a nicer job as well. You can check them out while off as well making sure brushes, drive, etc are good. Service as required.
Best of luck.
Larry Shepard