Hi,
Did this last week on a 79 RRC V8 - had to buy a socket for it.
It's about a 33.5mm - so a 34mm socket works (Approx 1 3/8") - make sure it is quite a deep socket to go over the starting pawl.
Also if the pawl is damaged, as our one was, run a file around it's outer edge as it's diameter is almost exactly that of the distance between the flats on the bolt itself, so any damage may prevent the socket fitting over.
If the engine is out of the car and separated from the gbox you can use a plate with two holes to tie the flywheel to the engine casting to prevent the pulley from turning.
Cheers,
Andrew.