[donut]

-I'm assuming that the carb float level is correct or a tad low?
-That ignition timing is correct?
-The advance springs and weights you used were plotted and not just thrown in because "the internet said so".
-All 8 cylinders are firing? (for that matter, why keep the dizzy that was set up for propane instead of replacing it with a new one?)
-break in was properly done.
Set the carb back to stock, iirc your about 2 stages leaner now.

Take truck to engine shop, ask them why would those two cylinders are acting like that. One of the techs should be able to diagnose the problem (compression, leak down ...) and decide which direction to go.

This way, you both can decide whose problem it is. (if it's not an engine problem, they can probably test it cheaper than you can.)