Hi there. I have fitted a range rover power steering to my 64 series. I have also fitted a 200tdi engine as well. If your steering is very heavy, I would suggest removing the two arms on the steering relay(upper and lower) and seeing if the relay has seized up. This is a common problem. Mine was 50 years old and seized tight, I had to use a three foot pair of stilsons to move it. It's a cheep replacement and easy to do.
Good luck.
Kev