Well, I suppose this'll get deleted.
1. Did you have VENTED plugs in the front of the block--the two plugs at the front of the lifter galleries, behind the timing gear? If not, next time put a ~.032 hole in each one. GM is doing this on the 5.7 Vortec engines, and likely others as well. The p/n for Melling vented plugs to fit those blocks (880) is MPC-52V. Melling sells other sizes of vented plugs, but it's easy to drill your own.
Other blocks might take a different-size plug.
2. SOAKING lifters does NOTHING useful. You wasted two days. Cold oil WILL NOT enter the lifter unless you pump them.
3. While there's a long, sad story involved, NOT pre-lubing lifters was the first of a chain-reaction of failure that cost me an Olds 455. Since then, EVERY set of lifters I install gets pumped full of ATF. I can see that the lifter will pass oil to the pushrod seat, I can verify lifter leakdown rate. And they're reasonably quiet on start-up. The engine does NOT run "really crappy". I do have to be very careful when adjusting lifter preload. The adjustment has to be done slowly to give the lifter time to bleed down. If not, you'll open the valve a little bit as you crank the rocker nut down--and potentially tag the piston, especially if you're not familiar with finding "zero lash". This could bend the valve. Slowly is good.
I am on the verge of saying that I'll no longer install lifters without pulling them apart and cleaning the dreck out of them. USED lifters WILL get taken apart. NEW lifters...I'm on the fence. I haven't had problems (since I started pumping them with ATF) but I'm hearing a lot of stories about extended run-time before they quiet-up. That disturbs me.
4. There's something to be said for pre-heating engine coolant (easy) and engine oil (also easy) before firing an engine after reassembly. Heated oil will flow faster into the tight-clearance areas, and if there's any hardened assembly (anti-corrosion) waxy crap in the interior of the lifter, it'll melt it out faster. Every engine I build gets a block heater. For typical Chevy-Ford-Mopar iron V-8s, they're about $30 and simple to install--when the engine is on the assembly stand. A magnetic heater costs more, but can be transferred from oil-pan to oil-pan as you build a series of engines. Not near as efficient as a block heater, but they'll warm oil eventually.
Wild guess based on zero evidence: Your lifters suddenly
got quiet when the gunk inside got warm enough to liquify. Until then, it was either interfering with the check-ball and seat so that the lifter couldn't pump up, or blocking the oil exit path trapping air inside.