Since you're not building an ultralight, I'd choose a long action. The reason is simple enough, a long action allows you to seat long bullets so that other than the boat-tail, they don't intrude upon the powder capacity of the cartridge. The manner in which I'd approach lead length is to seat the longest bullet you intend to shoot in the case, so only the boat-tail, if there is one, extends below the neck, determine the length from the base of the cartridge to the bearing surface of the bullet, then add 1.5 thou. If you prefer to index your bullets with a cannelure, base the lead length on the length of the bearing surface ahead of the cannelure, plus 1.5 thou. If you prefer to touch or jam your bullets, disregard the 1.5 thou allowance.
Give some thought to your barrel length and contour. A #3 barrel provides a .63" muzzle diameter at 24". A .284 bore rifle benefits from a long barrel, but unless you hunt open country exclusively, a 26" tube can be a drag. A 22" barrel is often equated as a good length for general hunting purposes, but means that you'll give up a bit of velocity compared to a 24". IMHO, a 20" barrel provides the ultimate balance between portability and ballistic potential, put probably shouldn't be considered unless the rifle is going to be used in heavy cover. I prefer fast twist barrels, so would opt for a 1:8. Even if a 1:8 twist is not necessary to stabilize your bullet, high rotational velocity will enhance the terminal performance on game, since precession (yaw) at impact is more rapidly overcome. Most importantly, consider the balance of the rifle when considering the barrel length and contour and the stock you choose. Your gunsmith might have a computer program that predicts the balance of the rifle given these variables, and his insights from years of experience is invaluable.
Be sure to have your rifle bedded correctly, a poor bedding job will have you throwing a tantrum at the range.
Keep in mind that every aspect of a build is a compromise. What you gain one way, you'll lose another.
