The problem is knowing where in the amp's output range the "safe" range ends. Due to the logarithmic nature of audio volume, the wattage goes up way faster than the dB's. The difference between your cab's 60 watt rating and the amp's 325 watt rating amounts to only about 7 dB of power difference. Not as much as many might expect. For comparison, a 10 dB increase in power is twice as loud. Exceeding the speakers rating by a full 100% (120 watts) would amount to only 3 dB of volume difference. It's easier to overdo it than many people think.
Using a good SPL meter might give you a rough idea of the amp's power output if measured correctly. Vintage 30's have a 100 dB sensitivity (with 1 Watt 1 kHz sine wave input measured at 1 meter away), so they don't take much power to get ear splittingly loud. At 60 watts they would be outputting roughly 118 dB. In theory, if you never exceed 118 dB of SPL from one at 1 meter away, you should be within the V30's rated range. That said, speakers are not perfectly linear in the way they react to power input and other factors like room reflections can skew your measurements so you might still want to err on the side of caution with such measurements. Keep in mind this also applies only to measuring single speakers, since multiple speakers in a cab would have a different combined SLP at various frequencies.