Re: .300 Win.mag./H-1000
H-1000 is my go-to powder for the .300 Win Mag. I am running 83.5 gr H-1000 with the 180 TSX, getting 3150 fps. I'm sure your load is safe, it's almost always safe to substitute a lighter bullet, and definitely safe to substitute one of the same type and brand, but lighter weight.
They shot great for me, all the way to 1030 yards.
The Barnes bullets, being solid copper- therefore harder than lead-core bullets- produce less pressure, you can pretty much always use more powder than with a conventional bullet of the same weight. You really kinda have to, in order to reach that "happy place" where you get a good, crisp load.
Full throttle is where my .300 likes to run, your results may vary, use caution, work up to max loads, don't run with scissors, etc.
PS, alot of benchrest shooters like H-1000 in the .300 WM