Because the mag box is a tad over 3 inches, I would go 260 or 7mm-08.

I have a model 20 in 284 and I had no issues finding brass, I bought Norma brass here: https://www.bullets.com/products/-284-Winchester-Brass-Cases-100-Count/BL11927 and it will last a very long time, but I also have necked up 6.5x284, it very easy, just a pass through the 284 sizing die.

Enough already, just shoot it!