Define “major mods”.
Will it bolt up to the block? Yes. That’s about the only thing that does.
The Rover motor has a different crank flange “stick-out” distance. The Olds doesn’t use a conventional flexplate, it uses a flywheel with a spring hub and spline drive like a manual trans. The Buick flexplate bolts up but will not work with the Rover converter. The Rover trans is much larger than the RH5, so you will need floor pan surgery. The Rover trans uses a flange drive with a rubber donut, if I recall, so you’ll need some sort of custom driveshaft. I don’t know where they put the slip joint. Also, the early 4HP automatics were fully mechanical, but the later ones use solenoids and an ECU like the 4L60 to 4L60E modification.