I had a problem like this one time on a small block chevy. All new plugs, wires, cap, rotor, good compression on all cylinders, including the one that was not firing. Turned out to be a broken valve spring. The SBS has an inner and outer valve spring and the outer was broken in a place that couldn't easily be seen. The inner spring could close the valve at cranking speed so the compression looked good, but could not get it closed when running. I believe the 450 also has an inner and outer spring. I know you just did the valve guide seals but maybe if you just pulled the springs off, set them aside , then reinstalled you may have missed a broken spring.
Just a thought.
The way I found this problem was I removed the valve cover and was just looking and thinking. I just used my fingers and pushed down on the 2 valves for the bad cylinder. One of them actually moved down a small amount with just that little bit of pressure.

PaulM

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84 500 SEL (307,xxx miles)