We need some help in trouble shooting. I'm really hoping HotRod Lincoln comes to the rescue again.

We've got a '51 1/2-ton with a '54 235 that is only running on 4 cylinders. The #3 and #4 cylinders are not firing. Here's what we've done so far.

Check the compression:
1: 165
2: 160
3: 165
4: 165
5: 160
6: 165

New plugs. We have swapped plugs from different cylinders with no change.
New wires. We have swapped wires from different cylinders with no change.
New cap and rotor. We rotated the cap with no changes.
We checked that the bushings in the distributor are tight and there does not seem to be any wobble in the rotor.
Since #3 & #4 share the same intake, we checked the manifold bolts and all is tight.
This is a new truck frame off so we even pulled the carb off the manifold and stuck a dental mirror down the throat to make sure we didn't leave a rag in the intake.
We have adjusted the valves.

Now what? Anyone else run across this?

Thanks.