Hi,
The voltage you measure is there because the insulation between the mosfets and the heatsink act like a small capacitor. A small current can run across a capacitor if the voltage is AC. The capacitance acros the mosfet to the heatsink should be very small and only a few micro Amps should run between them. If it is more the insulation you used might not be suitable for this purpose. Mica is a good insulator that can handle a good bit of voltage too.
You were lucky to survive touching the heatsink though, if the insulation was not up to the job you could have been electrocuted. The mosfets chop the AC current from the mains to a higher frequency to convert the AC mains more efficiently to 5V .. 12V than a regular transformer could. The result is that some of these mosfets are directly connected to the mains voltage, also the metal plate connected to the mosfet (very) possibly carries full mains voltage.
The voltage of about 65V is in the middle between 110 mains, suggesting that one of the mosfet is connected to 110V and the other to 0V. The capacitance of the insulation between the two mosfets act like a voltage divider and the resulting voltage on the heatsink is 65 V.
0V ---||---heatsink (65 V)---||--110V
You better make sure these insulator pads are made for this job...