Ford say you can do this without taking head off , using special tools !!! but thay are not that special , they say use compressor to feed air into cylinder at bdc , and the compress valve spring using a couple of special tools , do you know anybody in a ford workshop , will see if I can scan the workshop manual and send if you like so you can see ,I have access to it from home let me know .

You could try doing it at tdc and make yourself a lever bar up to compress springs with a cut down tube .