Before you go off buying any rover v8, do some research on which one will be the most suitable. There are variations in fittings, sizes, driving characteristics and so on depending on which v8 rover you get. For examplethe RRC engine had a heavier flywheel than the .sd1 engine so suits offroading. You will need a conversion kit, check ebay, and expect a bit of cutting of the bulkhead perhaps as well as possible cutting of the crank to marry up the s3 transmission. A conversion ring and drilling of the flywheel may be required. I have no experience of this, i just read the forums, just Google it there is amongst lot of info. It's not straightforward for someone like me but if you have fitted engines and welded a bit it should be ok.
Exhaust is the biggest pain as it will foul on passenger side. Apparently.