the diaphragm arm will have a number stamped on it indicating amount of advance breaker plate will move when vacuum is applied. dual point means you will have two sets of ignition contacts on the breaker plate instead of only one. this set up improves dwell angle of coil saturation.

Anyone know what the difference is between these two? Mine is an automatic and I haven't been able to find a 3620704 but have found a 3514265.

The only difference will be the amt of (total) adv the can offers (& it'll be stamped on the arm). You might be able to lookup the two different numbers (amounts of adv) & likely can live with/use either one and depending on if the number of the one you get is more or less than your orig bad one you can alter the amt of adv. decreasing it is easy, increasing it is hard(er) but can be done. this is a seperate mod than adjusting the tip in with the 3/32" allen wrench. EDIT What number is on yours and have them check what number is on the one that they do have (& post em)

As I try to get this thing tuned, another question is where is the best place to get a good intake manifold vacuum reading? I've tried the port on the carb for the vacuum advance and there is little if any vacuum there which could also be part of the problem unless that is normal? What should it be? Stock car, stock carbs stock cam etc.

The vacuum advance port on the carb is ported vacuum,you won't get a reading there at idle,Take it off the PCV port on the carb or directly off the intake.If "nothing" has been modified you should have at least 17" or more. Also,

Incidentally,the Prestolite advance you posted above is for a big block,this is the correct small block advance.

What are the differences?

Big block counter clockwise rotation,small block clockwise.The arm would be upside down and won't connect to the plate.

Pulled my cap to take a look and used vacuum pump to help see what's going on. The VA doesn't do anything until about 8 and doesn't move anymore after 15. It's kinda jerky too and not smooth in its operation.

Here's a short video that kinda shows what it's doing.



http://vid338.photobucket.com/albums/n439/kna4977/Mobile%20Uploads/IMG_0219_zpsprims56u.mp4

that is normal, you're in good shape (on this)

I would like to see what number is stamped on the arm but I can't see it with it together. I've never taken one of these apart before how does it come apart so I can see the arm, springs etc.? Is there anything special about it as far as disassembly? Is there a way to know how much advance the distributor has other than figuring it out with a timing light? I've set my initial timing to 10 btdc and I've revved it up to 2000-2500 rpm and it seems the timing wants to keep climbing beyond 36 so I didn't go any higher to see where it would stop going up. I'm used to GM distributors they don't look like this.

If you look down behind the condenser you can see it but it's not easy,it's probably stamped 8.I have four Prestolite 340 distributors and all stamped 8. The one I posted above is off an original 340-6 AAR.

OMG please replace that burnt POS rotor
you need an original style hd body,riveted
real steel contact

Finally getting around to replacing the vacuum advance. I was unable to find an NOS canister for the automatic but got an NOS 3514265. They look the same but the arm on
my original is an 8 and the one on the new one is 8.5. What do I need to do to set up the 3514265 to work as it should for my car?

I doubt you will notice any pinging from the addit'l (1) degree you'd see on the crank (8.5-8=.5 x 2 = (1) deg. You can JB weld shims on either side of the notch in the arm on the case itself to limit the arms' travel & experiment with thickness till you see 16 addit'l on the crank when you plug in the can (you'd have it idling & plug the can into manifold just for this situation) assuming the chain and intergear slot/dist notch is tight so the mark ain't dancing around at idle so you can see exactly if the can is giving you exactly 16 (8x2)

By chance does the diaphragm adjust with an Allen wrench to limit advance?

CCW with a 3/32" allen wrench slows when (in hg amt) the curve amt (8x2=16 degrees) starts by tightening the spring so it takes more vac to overcome that tension & there fore at what amt (in hg) the curve ends so in effect once you are in the curve by tightening the spring (CCW) you have less adv at any given in hg amt so likely that would quell any pinging if it did ping exp cuz we are very close on the amt. you would monitor this in eveyday driving under varying RPM/throttle (load) positions