Can I fit my motor and tranny in a 96 Deville . It's coming out of a 95 Deville , if so what do I need to do or look out for ?

I have the 4.9 V8

The 1996 Deville did not come with the 4.9L V8 as an option at all. Therefore, I would consider this engine swap not possible without major changes in wiring harnesses, processor swapping, and so on. If the engine had been offered exactly year to year, I would say it is possible without much drama. But it absolutely is not.