Peter,
There is a chap in Maldon, Essex who is one of, if not the UK's best authority on the ZF, he rebuilds and converts (flips) them for historical racing applications and has been at it for 15 yrs or more, the last contact details I have for him are e-mail; paul@flemingpaul.wanadoo.co.uk mobile: 07710 778672 Tel/Fax: 01621 854950
He would be a good go to in the first instance.
Julian
PS Here's a photo of my GT40 for inspiration...