Not using a timing light makes it tough to get an accurate and repeatable tuneup.  I’ll suggest re-curving the distributor so that you have more initial advance while the total advance without the vacuum advance hooked up is in the 36-38° neighborhood.  For this, you do need to get some timing marks on the damper so you can use a light.

Lorena, Texas (South of Waco)

The damper does have marks.  They can be difficult to see.  I used some white paint on the tip of a sewing needle to highlight all of the factory marks on mine and add marks up to 40 degrees.
Damper marking is handled in two ways I am aware of.  Most have the marks on the damper.  Some have the marks on the pointer, multiple points on the pointer, with only one line on the damper.  If you have a mismatch of damper and pointer, then you can have difficulty with the timing.

With the vacuum you have, the factory powervalve should not cause a rich condition under normal driving conditions.  I have a 570 had had to jet it up quite a lot but my engine is not stock and according to Holley's chart, I was right on the borderline of a 570 or 670, which probably has something to do with it.  Run it out of the box and give it what it needs from there.  Just tune it correctly don't try any funny stuff like you're hinting at with the secondary spring.  I get mid 16 highway mpg which is not bad at all for a truck.



Lawrenceville, GA