Since I have the cluster torn apart, I'm wondering if there's anything I can do to make the speedo run smoother without bouncing like crazy at low speeds. The cable is all lubed up and turns without any resistance at all, so I'm wondering if this is something that can be done with the gearbox.

Would the gauge still be accurate if I tightened the spring that pulls the needle back?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

My needle is also bouncy below 30mph, and sometimes it doesn't work at all so when I get around to it I am going to replace the entire speedo. It is magneticly based and I have a feeling it just lived its lifetime. I'm thinking of going electronic.

Jopapa
You will loose some acuracy. The spring is to recalibrate to proper MPH reading.
Your bouncy state probably comes from the plastic bushing on which the shaft from the bell to the needle moves. Check your bushing, it may be broken not worn out.
I had this happen to me.
Rafa

Where would I be able to find a replacement bushing?

What about the magnetic system that Pierre mentioned? The cluster is the standard A-body cluster from a 1970 Dart Swinger.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

System is not magnetic, at least it's not in the odomoter of my F-bodies. It works on vacuum created by two "winglets" that move around the aluminum bell at the end of the shaft.
Only fix is getting another odometer from a similar cluster, or if you are good at fixing tiny things you can try a drop of crazy glue (applied with the tip of a pin, not directly from the bottle) to the bushing.

Rafa

Hmm I could be wrong I thought it was magnetic because the needle was attached to the mentioned aluminum bell, and I thought I saw small magnets on the "wings" that spin in the bell. I figured the faster the wings spun the stronger the .... Wait a second, if the bell is aluminum, aluminum isn't magnetic. Whoopsy, ok toss my theory out the window.

Magnets moving near non-magnetic metal (like aluminum) does weird things. Look at the electric meter on your house. See the aluminum wheel spinning? It is turning because of the magnetic field from the AC current in the windings in the meter.

The magnet in your speedometer spinning around the aluminum disk causes drag that makes the needle move up.

Check out this website about the super strong Neodymium magnets....

http://www.eskimo.com/~billb/neodemo.html

It mentions some experiments you can try... See specifically MAGNETIC BRAKING ; FEEL THE DRAG DIRECTLY ; VISIBLE SLOW FALL ; and
HOVERING SLOW FALL.

Have fun with your magnets!