A 284 cartridge must be ~3.100" or longer with 162amax and 175smk to have the bearing surface ofthe bullet north of the neck/shoulder junction.
2.950" is the practical maximum in Alpha magazines.
Lapua brass can be used. It works great for the for the first firing, but 50%+ will develop a donut the first shot. The rest will form donuts eventually. That can be fixed with neck turning and/or reaming if you're inclined.
There's a reason they're called the "dreaded" donut. They suck. Best bet is to address it before ever firing. Neck from 6.5-->7--7.62, neck turn and get into the shoulder a bit, neck down to 7. You'll have to sacrifice a few cases getting the shoulder cut right.
This applies to my chamber, which is a PTG "match" with a .317" neck. Speaking of which, .317" is OK for Lapua but too tight for WW if you don't plan toneckturn. I recommend .319".