Jason, thanks for your thoughts. Yes, it's the original updraft carb as I mentioned in the first post.
I don't think the problem could be intake restriction; if the intake was over-restrictive, it's likely any problem would appear at high revs when a lot of flow is needed. The problem here is the opposite; it wouldn't idle but would run at high revs. Even a very restrictive manifold should flow enough air for it to idle.
As I say above, although it hasn't been back on the road yet, with the carb rebuilt the truck immediately idled smoothly and picked up cleanly, so it's likely the multiple carb faults mentioned above were the problem.
