Ring gear lock. Bolts in place of the starter and engages in the ring gear to lock the crank.
I'd guess others might suggest a similar method via a screwdriver jambed in there somewhere, but there is correct geat designed just for this job.
Crank bolt on the GTR is 340ft lbs so you must have the right gear for that job.
Have a good look at your balancer. If the centre hub comes right to the front, (no rubber mounted pulleys ahead of it) you could use the Toyota 1 and 2JZ idea where they bolt a plate hard up to the front face of the balancer.
The plate has a large hole in the centre to tension the crank bolt. The plate also has an arm welded to it so a convenient length of pipe can be slipped over to hold the whole show while you tension that crank bolt.
Attached is one made for an auto 2JZ. Note how the centre hub comes flush to the front so you can bolt this plate firmly to the balancer without damage to the rubber mounted pulleys.
Typical Toyota thinking, nice large diameter puller holes too.