HB, I have the same rifle. Mine is the later model stock. I just love those things for the money.
Looks like my Hornady 4th edition was off about 90fps on your H4350 loads. 280Rem was almost dead on!
Your gun may not be able to take the max of 55.1gr, if it does you could try 54.5 and 55 to see if your groups tightened back up.
Scrolling down to IMR4350 Hornady has 57gr as max, sounds really warm to me. Nosler is at 52.5gr for a 140. That is a huge spread. Anybody loaded IMR4350 that hot?
