49N, your rifle would have surely used a 480gr bullet and a charge of 85grs of powder. The 540/75 load was a target shooting type load in the No.1 cartridge that pretty much replicated the performance of the 577/450 MH. Your rifle being a sporter and not a MBL or match rifle, I would think the 480gr is it.
From my reading, the 360gr bullet was introduced well after your rifle was made.
That said, many of the later Gibbs sporters were sighted for both the 360gr and a 570gr bullet and shoot both well so dont get hung up on your bullets weight too much to begin with. The Metford rifling must be pretty forgiving in respect to bullet weight variation.
