5.7 Vortec intermittent long crank --> crank no start, backfires, clunking

[slow_c1500]

Update: trucks running fine, still has the intermittent long crank tho. I know a lot of you are suggesting that its the check valve in the fuel pump, but if the fuel pump was the issue, wouldn't that mean that priming the key a couple times would make it start right up? Cause priming the pump beforehand doesn't help.