I can’t tell you what the problem is without truck in hand. However, I can make a couple of comments.
You might check for a clogged converter. Testing with a vacuum gauge is the easiest and best method IMO.
You say it had a cracked exhaust manifold. A plugged up converter can cause severe overheating and even turn the manifold red. It can also cause rich running and sooted up spark plugs.
As for the timing, I suggest you do not run that much advance until something is sorted out. Excessive advance can easily destroy the engine; especially at sustained highway speeds.
I’ve seen more than a couple of engines wiped out because of something like that.