Nitrox,
Sorry to be a bit anal here, but the course of fire you have is slightly incorrect.

It is -
50 metres 2 rounds offhand in 5 minutes
50 metres 2 rounds rapid fire offhand in 10 seconds
25 metres 2 x 2 rounds rapid fire offhand in 10 seconds for each 2 rounds.

Please remember that in the rapid fire events in BGR you can start with a round in the chamber and cocked, ready to go. I shoot Blackpowder express with a 450 NE in a Ruger #1, 10 seconds with a #1 is more than enough.
The most popular Group 3 rifle in QLD amongst our shooters is indeed a Ruger #1 in 500 NE. As a bonus, you can also double up and use it for Blackpowder express as well, 2 events with the one rifle.
Personally, I shoot a bolt gun in 500 Jeffery, but I had it before the 450 NE, otherwise I too would have built a 500 NE.
Build it up to about 10 lbs and you will not have any dramas shooting it, 75 kgs or not. We have a lady in our club that shoots a 510 Wells that weighs about 11 lbs, she would not weigh any more than 60-65 kgs at a guess.
Recoil tolerance levels have very little to do with what you weigh.

FYI, you'll need a 535 grain bullet doing a minimum of about 2150 fps to make the energy levels.

Regards,
David Commens