Rocker arms

Are you familiar with how to adjust the valves on older GM engines by centering the lifters in their range of adjustment? This engine is not adjusted the same way. According to GM, there is no adjustment for this valve train, so I suspect what is happening is the lifters got pumped up all the way, and they're not bleeding down by just sitting there. As long as the engine will run, gradually tighten just one bolt about a quarter turn at a time. You'll hear and feel the misfire when the valve is being held open, but the lifter should bleed down and the misfire should clear up within a few seconds. Once it does, tighten the bolt another quarter turn and wait for the misfire to clear up. Don't do that too quickly because the valve needs to close so heat can be transferred from its face to its seat to prevent overheating that valve.

Once you have that bolt tight, move on to the next valve and do the same thing. You can torque them all to specs later once they're all snugged down.

Yup. That's exactly what they do in normal operation when the parts warm up and the push rods and valve stems expand. It shouldn't take more than 20 seconds for one to bleed down to self-adjust, but when tightening the bolt a quarter turn, you're asking it to bleed down a lot more and much faster than in normal operation.

Some lifters will not bleed down very well when steady pressure is held on them. That could be why they didn't do that when you first bolted everything together. They need the push and release of the cam lobe to let some oil out, and of course, that means you have to get the engine running first.

Please do post the solution. That can help others who are researching the same problem.