nah... nothing to worry about.

Have you replaced the right angled adapter for the oil feed from the block? If the inbuilt restrictor is removed, the lifters get less oil pressure and make lifter noises...

The lifters should have been kept vertical, and stored submerged in clean engine oil.

The lifters get oil into them from the sides, and the little hole in the top is what lubricates the cam lobes. If any of the holes are blocked, the lifters won't pump up.

Getting new ones is not too expensive <$200, get a quote from Nissan.

Before removing the cams. turn the engine over by hand and measure the clearance. Note which lifters have the biggest gap between the heal of the cam and the lifter.

Cleaning the old lifters (not easy to be successful), and making sure your valve springs are in the correct way is the best starting point.