Hello Peter,
I'm in the process of changing my son's timing belt tonight on his 1989 R32 GTS RB20 manual. I put the car in 4th gear , handbrake on . Used a 27mm socket and breaker bar. It come undown quite easily. So yes I think you will have to positely lock the engine somehow. I did read an article where this guy wrapped a peice of twine around the crank pulley and the alternator pulley. I'll see if I can find the picture. It may be of some help. I then tapped out the two 6mm bolt holes in the pulley and used a puller to remove it.
Regards
Brian