Holy Sh*t Batman! A Chevy in a Rolls! Diabolical
I've seen my share of these. Some run like a raped ape, others just ruin the car. As for ease of swap due to the same trans, the bellhousing is totally different and NOT interchangeable. Same guts, different bolt pattern. Chevy has one bell, Buick Olds, Pontiac, Cadillac have another, RR, Jeep have another. You can use wild torque converters. They do interchange.
It's most likely cheaper to get a used RR motor and swap it for a worn out one than change to another engine. Too many issues, especially for the brakes. The Citroen belt driven pumps will solve the fluid pressure problem, but logistics add another layer of BS to deal with. For performance, a well done 6.75 V8 will kick some serious tail. I can help with that one.
Good luck
MB