Yes, it's been done and Mike Piggin was probably the first to do it, more than 10 years ago. Recently, Les Keeling has fitted one to his car and found it to be an excellent modification. I drove a fifth series car in Australia which had an overdrive and I was very impressed, and I intend to fit one to my Lambda. There are quite a few in Oz with them fitted and the general concensus is that the Laycock "J" type is the one to go for. These were fitted to Triumphs, Volvos and even Ford Transits! I found one on eBay for about £100 and spent a further £300 or so on suitable a adaptor plate and drive to fit the Lambda. There's a firm specialising in fitting them to vintage cars, but they are quite expensive. Take a look at:-
http://www.nostalgiacars.co.uk/overdrive.htm or for spare parts:-http://www.odspares.com/ where you will see thew general arrangement. I still need to get round to fitting mine, but have to mend the Augusta first!
