I strongly lean towards doing a thorough prime on a fresh build to get the lifters pumped up to the proper height instead of priming the lifters individually out of the engine. Run the oil pump and slowly rotate the crank 2 full rotations, if your lifters aren't filled by then there's probably something wrong. In my mind that's the least-risky way to get the engine primed and ready for breaking in a flat-tappet cam.
You can also just rotate the the engine until your on the firing stroke of each cylinder ( both cam lobes on their base) and prime the oil system while moving the pushrod side of the rocker down and up with your hand.
