Gonna throw some old fashioned hot rodder stuff into this...
In the 80's (and 70's) there were a lot of wierd things done in the name of low emissions, that may or may not have affected truck engines....
In an ideal world, a chevy engine will have 16-18 degrees of initial timing, 32 to 36 degrees of total timing (measurements taken with any vacuum advance disconnected and plugged)
Vacuum advance will provide another 12 to 16 degrees, and for best performance it will be on full manifold vacuum. Hot,out of gear advance at idle will be well over 30 degrees, and cruising advance could be 50 degrees or better.
In no cases should there be enough advance to cause pinging.
During the 70's and 80's, they did a buncha wierd stuff with timing. "transmission controlled spark" that prevented full advance until you were in high gear.
Ported vacuum advance that prevented full advance until you had some throttle in it.
And generally lots of stuff like this that did weird things to the engine in the name of keeping cumbustion temps up and minimizing NOX emissions.
So, if it were me...
I'd remove all those wierd gadjets and vacuum lines.
Connect the vacuum advance to full manifold vacuum.
Recurve the mechanical advance to provide 16 degrees at 2200 rpm and zero at 1000 (this is important, most engines in this era had way too much mechanical advance to compensate for having way too little initial and vacuum advance!)
And set the inital timing (vacuum advance disconnected and plugged) to 16 degrees.
