Better yet, why don't you try setting your dialog norm to 0dbf. Since the audio should have been normalized correctly when they made the ac3 for the DVDs, in theory keeping it the same would result in the same level when you mux it back into ac3.
I don't think that will work. The Doom9 guide says quite clearly that if you don't normalised it close to -29 to -31db ish, it can result in DRC occuring in the wrong place which can result in incorrect pumping and clipping of the track.
what pisses me off is that we are dealing with an already normalised track. they should put in a bloody option for it to be encoded as is, ie just convert it to AC3 without pissing about re-normalising and DRC'ing the wav's.