My buddy's 350 mag was a Rem 660 mag with a 20" barrel. We shot a lot of cast bullet's thru it because it was a heavy recoiler. I don't think the 20" barrel was a draw back at all. In fact my own 308 win in a 660 Rem was one of my all time favorites. Your talking the same case I think and won't be using all that slow of powders. A rifle with a 20 to 22" barrel handles really well.
Speaking of that old 350 Rem, with cast bullets it was a lot of fun to shoot an I suspect it would have made a dandy woods rifle/cartridge with those cast bullets.
When I lived in Alaska and until I got over the fear of a bear behind every tree, I thought I'd really have liked to have that old cannon. That had to be about a perfect dangerious game gun. Short, quick handling, powerful as hell and,,,,, well,,, some people thought it was ugly, noy me!