Did you re route the ignition wires exactly like the TSB shows?

Have you replaced #8 ignition wire and spark plug with "known good" substitutes?

Like i wrote in your previous post
looking at all 8 spark traces on an old Sun machine display
could eliminate doubts about ignition.
Those old Sun machines could also do a "cylinder power balance" test at 2500 rpm.

Looking at the valve springs, rocker arms and hyd lifter bleed downs is quite reasonable, as Magnums have been known to have problems in all 3.
A video boroscope makes this inspection quicker and easier but removing valve covers is also worth the time and gasket costs. Looking inside all 8 cylinders with the video boroscope through the spark plug hole is worthwhile. Are there deposits on intake valve stems?

Having down the static compression test on #8 is good. Your 155 is typical.
A "running compression test" also called a dynamic compression test
would also be instructive in spoting a bad valve spring or lifter,
and could even give a hint about intake manifold runner air distribution variation cylinder to cylinder.

Have all the previous worries about the Carb set ups been eliminated?