There are a number of variants of the hypothetical designs for a P1500 weapon, Matuo Kasten has gone for a slightly different approach of having what appears to be a "Dora" transported on a pair of "Ratte's". This is not so far off the mark (if such a comment can be sensible placed whilst discussing such a crazy idea), as there was similar talk of mounting a "K5" Leopold between to (King) Tigers? Therefore scaling this up in a logical fashion gets you to the proposal shown above.
As Matuo Kasten has previously produced two versions of the RATTE, I think we can safely assume that this will get produced. The questions is do you have a budget and a shelf big enough to hold it!
Keep Track at; http://matuo-kasten.jp/