I think last gen D-body went 93-96, but in any case the trunk is larger in about all dimensions. From a FWB to a B-body should be doable with trimming here and there. Can't vouch for going the other direction. There's a portion of the sides molded to conform to a diagonal reinf. strut backing the quarters and/or the elec. ant. in the Cady that the B-body don't have.
Don't bother. Been there tried that. You will spend so much time trimming and altering the Fleetwood carpet to get it to fit that it will not be worth your time and STILL look terrible when you're done.