Need help balancing tonearm/ heavy cartridge

Tack on some additional weight, BluTak or putty of some sort should work.

As Mofi says, get a lighter headshell. the ADC Magnesium shells are light, as are the vintage audio technica (e.g. ls9) and sony ones (I see an 8gr one on the bay right now). Also, some of the Yamamoto wood headshells are light (e.g. the boxwood one is 8gr). Maybe also change your screws to alu ones if you're not off by much.

If I understand the problem correctly, the solution might be easy. The only important thing is the tracking force. Assuming that you are using a scale to set the vtf, it is not necessary to balance first. Balancing is only important if you are using the arm's built-in scale to set vtf. As long as the rear weight difference is less than or equal to the tracking force, just use your scale.

Bill

You could also try mapleshades tone arm resonance kit.