davem3:
Before doing that, might I suggest you try a heavier bullet. That 365-grainer is on the light side even for black-powder loads, the mainstay of the .500 Express was a 380-grain bullet. Nitro-for-Black loads generally require a heavier bullet to shoot to the sights. The 'correct' weight for that cartridge would be 440-grains.

There's a better-than-even chance that a heavier bullet alone will solve the problem, but you may need to adjust the load slightly to regulate.

Graeme Wright's recommended load for the .500 Nitro for Black cartridge is 72-grains Varget beneath a 440-grain bullet, with a polyurathane-foam wad to take up the space.
He's usually 'wright' on the money!