.6v will not kill the motors. I have run those motors on 6 V before without any imediate damage( maybe some long term though...)
But an extra .6v will definitely not do anything to the motors except make it run faster.
Now why do you need a heat sink for?
You're dealing with very very low amperage here. If it was a 10 amp motor than maybe maybe if you wanted to be safe you can add a heatsink. But thats if you want the motors running continuous for a long time.
Summarized answer ( lol
)
Go ahead with the extra .6 Volts and no heat sink is needed.
Also when you buy a miniature gearbox it usually comes with a motor included. Search around and you'll ifnd it for cheap.
Also, where are you getting the treads from?
P.S. Those motors are very weak and you must be making a very small robot . If you're going bigger get a motor with higher rated torque of look for continuous movement servos( buy or modify an existing standard servo)
Hope I helped,
Eric