Misfires can stem from a number of things, but generally the diagnosis goes something like this:
1. Coil Pack - moving the coil pack from cylinder 8 to another cylinder, clearing the code and then driving the vehicle will reveal if the misfire follows the cylinder pack to the new cylinder. If it does, the coil pack is most likely the problem and will need to be replaced.
2. Spark Plug - if the coil pack doesn't present as the problem then the next item to look at is the spark plug. The same procedure can be done, swapping the spark plug to another cylinder to see if the misfire follows, but it's usually easier to just swap for a new plug to see if the misfire goes away.
3. Fuel Injector - The easiest test here is to ohm test the injector to see if it has failed, but injector failures can be intermittent so the next step is the same as the others, pull the injector for cylinder 8 and swap it with another cylinder. See if the issue follows the injector.
Beyond those 3 components misfires can be caused by corruption in the ECM causing the electrical system to not signal the injector to activate at the proper time. The lifter failure / cam lobe wear problem can also cause a misfire, but hopefully not on a brand new engine.
Were all of the components new on the engine or did they swap over items like the coils and injectors from the old engine to the new? Cylinder 8 is tucked up under the overhang of the firewall and is where the most heat in the bay gathers so the electrical portions of the injector and coil there take a beating over time.
