Couple of things:

First, look at the bright side: Glowing red manifolds make great cigarette lighters.

Second, Everyone is acting like it just has to be ignition timing(and I agree), but it could possibly be _injector_ timing. While I am not sure about Isuzu engines of this vintage, some engine's ECM use cranshaft position AND distributor information for injection timing. Discharging fuel just a few degrees late would send a lot of unburnt fuel out a partially open exhaust valve. COuld all of the distributor modules you have tried with these distributors be different enough fool your ECM??

Just throwing it out there for discussion since I don't know this particular engine well enough to know if this guess is even valid.

NCDiesel