[markeb01]

Another consideration depending on the age of the components, occasionally the outer ring on the harmonic balancer can slip making the timing mark(s) completely inaccurate. Choices are replace the balancer, or simply set the timing by ear, rpm, and vacuum readings. If this is the problem, keep putting in more advance until it won't start when it's hot, then back off a couple of degrees.

Also, when the truck won't achieve speed, is the engine racing as if the clutch is slipping?