I set my cars up similair to Steve did in the pic and use oven cleaner and one of those rinky dink Home depot press washers, they come out clean, but sometimes the undercoating will still look a little brown'ish from age..but hey thats real patina.
I also hit value village and buy 4 old bed sheets and lay them out wet them to stay put and put the car ontop of them, its catches all the crud and makes cleanup a snap when done