advance springs

I have an NOS distributor in the engine that is in the truck right now. Found it on ebay a year ago. Came to me in a genuine Mopar parts box with an old "from that day" parts logo tag on it.

I only have like 100 miles on it since I did the lean burn ectomy on it, and completely redid the underhood wiring harness due to past owner butchery.
I want to put that one in my new motor when I swap it out. But I want to rework it first, to better suit my engine as built, and considering that it's in a truck. I also have a good used core here to rebuild, And for that core I have the new parts I listed in an above post. The distributor gear I have here is a Echlin DG404, old stock, cast iron.... now a days, all that can be had (even by that same PN and from Napa) is nylon. I'd like to find at least 2 more cast iron ones.one for the NOS distributor, one for for my core, and one for the slant in my kid's truck.
In past years I've had problems of the nylon ones breaking. Not often but I've had a few.
Both my nos one and my core that I want to rebuild are Mopar PN 3874876. Which I think is the super six unit.
I haven't done anything with the nos one except to drop it into my engine and run it,
though guys on the /6 .org site had said it "should" have a vac advance can with "11.5" on the arm, but they gave me a part number for a STANDARD brand advance can with an "8.5" on the arm since I am using this engine in a truck instead of a light A body.sound right? Or not?
What difference would an advance can with "11.5" on the arm change vs one marked "8.5"?

I have 2 of the recommended advance cans here/one for the nos distributor and one for the spare I am building up. But have not yet put them in.
Regarding advance cans: on the ones that are adjustable, what does playing with the Allen head screw in the vac nipple actually change?
Is it the range where vac advance starts and stops, (by amount of vacuum applied, I'd guess), or is it the total amount of vac advance that can will provide, or something else?