If this is the type of maniac that tries to pull you off your hands alot, but isn't retarded, I would say his callingrange on flop is like 22,33,77-TT,some 23s, 67s,78s sometimes KsQs,KsTs (ofcourse this can include other flushdraws if he is really spewy)
This gives you an equity of: 43%

So.. yeah I'm probably wrong , still it's a big overbet so why not just bet smaller? (I'm really interested cause I've been frustraded in these spots before)