I borrowed a friends timing light to check my timing and to my surprise it was no where close to what factory specs call for, in fact it was a whopping three times the recommended 6 degrees of advance, it was set to 20 degrees advance! (I had the vacuum advance disconnected).

When reving the engine to 2,500 rpm the advance kicked in an additional 16 degrees where it topped out, for a combined total of 36 degrees.

My engine is a basically stock 300-6 with the exception of carb/intake/header, and it starts/runs/drives just perfectly. I temporarily set the timing to the factory 6 degrees and the truck was a pig, very sluggish! I then set it to 12 degrees, a little better but still sluggish.

Is it ok to leave it at 20 degrees? I've heard no evidence of detonation, nor did I see any evidence of detonation when I changed the spark plugs. 20 degrees of advance seems pretty high to me, but the engine runs great and I'm reluctant to change it.

EDIT: I should also add that we checked for true TDC and the timing marks were spot on, so the balancer hasn't slipped.

 

36 degrees total at higher RPM is in the right ball park, maybe a few degrees high but close. 20 degrees at idle sounds high... maybe what you would run with a lopey high po cam... but you didn't mention a cam change.

Since the initial advance is high and total advance is in the right ballpark, I wonder if the distributor has been modified? Any indication of welding on the advance slots for example?

Also the distributor advance mechanism may be sticking. I have found a shot of WD40 plus working the advance can free it up.

 

No modifications to the distributor that I'm aware of, and the advance mechanism appears to have total movement without any obstructions.

One thing I did notice when playing around with the timing was the idle screw on the carb was almost completely backed out, backed out enough that the throttle blades were completely closed. I assume this is to compensate for the 20 degrees of timing.

When I set the timing to 6 and 12 degrees I had to turn up the idle screw accordingly to achieve a decent idle, but then my total advance timing came up way short, 22 and 26 degrees respectively.

 

Nothing wrong with high initial timing. As you noticed it leads to a peppy engine.

Use a vacuum gauge to tune accordingly.