One perennial weak spot on them is the front stub frame. The back end of it is right under the front seat and it's typically rotted through just in front of the rear cap. Custom 880 trim is going to be the hard part; C880, Polara and Monaco all have the same sheetmetal as previously mentioned, but the only other thing common between any of the three on the outside is the grille and front end trim. Same thing inside, all basically the same but for the trim design and a medallion or three. That one looks better from the one pic than the 65 Polara convertible in my back garage waiting on me.
Mike