Ive shot 54x50 in a 1-28 twist and ive found a Rem CLHP 50-385gr to be about max. The sabot simply wont handle much more powder and recoil is brutal. I had to go upto a max load of BH209 just to get good groups and several MMP purples failed. The Barnes RED 54x50 sabots are tougher (never failed) but only come with Barnes bullets.

I would use the bullet Tony Knight recommended with a 54x50....The Speer 50-325gr HP or the Rainier 50-335gr if you want to save a bunch of money. $29/100 is a great deal IMO at MidwayUSA and its good enough for Alexander Arms to load in Beowulf ammo at 1900fps.