I've made several belly sliding landings, p38( meant P47 :-) ), p51 f4u
I try my best to get near 100 mph and as I touch down I pull back on stick and let the tail touch first slowly easying off on stick pressure to let whole fuselage touch down, any slight forward stick motion will prob dig in the nose/prop and flip you/crash you
I have had no problems so far though, with or without flaps. Only time I might foul it up is if part of wing shot off, or flap missing on one side etc.....