Finally had some time to try the troubleshooting methods suggested.
Here's what I tried:
1. Filled the chambers with water. No water leaked out of the valves.
2. Tried Cliff's suggestion to pressurize the intake/exhaust ports with air while water was in the chambers. I was able to get some minor bubbles out of the two exhaust valves on the dead cylinders, but as many said, didn't seem to be enough to make the cylinder have no compression.
3. Pulled the valves. Not an expert, but the valves and seats looked the same on the two dead cylinders as the the ones with good compression.
4. checked the pushrods, they look straight to me.
5. checked the cylinder walls. They look good, crosshatching still visible on all cylinders. Very minor scuffing on one of the dead cylinders... perhaps due to fuel washdown?
6. Finally, took Blue bird 68's advice to reinstall the head with the old head gasket and fill the cylinders with compressed air to see where the leak was. What I found was with the valves adjusted, I was getting air past the valves when they should have been closed.
I spend a lot of time watching youtube videos and adjusting the valves. Many guides online suggest tightening the rockers just so the pushrods have no up/down play/can't be turned by hand. Then tighten 1/2 to 3/4 turn to set preload. What I found was that almost any preload, even 1/8 turn, was enough to make the valves leak and drop compression to almost nothing. I was able to replicate this on any cylinder, not just the two that had originally had no compression. By tightening the nut on top of the rocker just so the pushrods stopped turning, and then locking them down, I was able to get healthy compression in all cylinders.
I don't understand why this is. Is there something wrong with my valve adjusting technique?
I do regret not doing a more through diagnosis before pulling the head... it seems like a simple valve adjustment with compression test to verify the valves were sealing would have clued me in on the problem.
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