Did head gaskets two years ago. Developed a slow leak on the pass side rear of the motor & I have been getting pressure in the cooling system very quickly. The coolant cap will release pressure after a min of running.

I did replace the cap to make sure it wasn't bad.
I pulled the head and replaced the gasket. The old gasket looked good. I guess my leak was from the valley gasket.

All the liners were flush with the block and the head had no marks indicating a slipped liner.

The leak is gone but it still builds pressure very quick and I am getting a P0304 code.

Next step is to try Irontite Ceramic Seal.

 

Being that it's not one of the end cylinders near the water jackets. I would say, there's a good chance you're correct. I have looked at cracked blocks in these trucks a couple times. In both cases, the liners looked flush with the top of the block. Both of these engines had misfires after sitting for a few days.
On one of them, we took it apart and cleaned around the top of the cylinder with the misfire. After sitting for about 20 minutes or so, you could see a little bead of coolant starting to form between the liner and the top of the block. It was pretty subtle, but we tried cleaning it a few time and each time coolant would slowly seep out. It was hardly noticeable. You could easily miss it while doing a head gasket job.