Passenger footwell needs modifying to clear the nearside exhaust manifold on the V8, a new exhaust system is required, a remote oil filter, and general ignition system will need to have the Rover V8 components fitted in place of the standard 2.25 components. The conversion kit should include a conversion ring to mate up the S3 bellhousing to the engine and also a set of engine mount brackets that bolt onto the existing standard 2.25 engine mounts.
You will need to possibly source an alternative radiator, particularly if the V8 had a long nose water pump fitted. The SD1 radiator fits quite nicely if you use a pair of metal brackets to hang it from the top of the front radiator panel. An electric cooling fan is more than likely going to be required as well.
That's just off the top of my head, there are bound to be many other small little details along the way.
