My 1987 Speer manuel lists 75g 7828 and a 275g bullet as max and 71g as a starting load if that helps.
Start at say 70g and work up for a 285g bullet.
Whoops! that was for the 338 Winchester. 340 Weatherby starts at 84g 7828 and max at 88g.
Maybe you could use the Weatherby max as a starter load?