The gear that is nearest to 1:1 is used, on some cars thats 3rd and some 4th. On 6 speed cars is more commonly 4th and 5 speed most of the time its 3rd. But thats not a hard and fast rule.

Most modern dynos can have the gear used for the pull in put in to the software to compensate.

Whatever gear is used I would think that almost any RR will provide enough load for the engine to reach its preset boost limit.