I fitted a servo (booster) to my Oval a while back and put a short article on the PG website (
http://www.performanceghia.com) but it is not easy and really shortens the turning circle the way I fitted it.
If I were to do it again, I would use Porsche pedals and use the system they use on the pre-89 911 where a link rod is used from the brake pedal to the master cylinder and servo which sits facing rewards infront of the dashboard.
MG