Be sure to uprate the front springs in light of increased engine weight. Parabolics are best. The Rover EFI is 3.9l in displacement and I think its slightly larger - it's going to be tight with the 3.5. You'd be wise to also put in a range rover box, and consider using 4wd as much as possible to spread the extra power to both axles - the series 1 didnt have the much stronger ENV or sailsbury axles of later models. Also make sure the V8 came from a range rover - the compression is slightly lower than on those fitted to the SD1, for example, and this is preferable for an off road vehicle.
Having said this, and given that the bulkhead will also need to be cut in to, I would suggest you put in a post-1980 2286 engine from the s3 (post-1980 engines have the much stronger 5 bearing crank). It's simpler, you won't risk breaking axles and the gear box, and it'll fit nicely into the bay meaning there's room to maintain it.