I don't very much about capacitors but I have a supplier that insists that a 40 mfd 370 VAC rated running capacitor can adequately replace a 40 mfd 450 VAC rated one. Based on what I read, a starting capacitor can vary by up to 10% but that running capacitors must be matched perfectly.

They are both 40 uf and operating off a 240 volt source. Just fine. Give him a beer for giving him such a hard time. He's right.

So that I can advise customers in the future, where on this capacitor does it say 240 volts? I see 450 VAC, 50/60 Hz

Well, since this is a DIY board, and domestic voltages are 240 VAC, so I assumed 240.
The voltage rating of the capacitor must be greater than the line voltage. Hence 370 and 450 V are greater than 240, so your OK. The higher voltage introduces a higher safety and likely the lower voltage has a lower ESR (Effective series resistance). This will not likely matter.

The start cap uf value is more critical than a run cap, if used. The run cap may change the speed.

Extension cords, for instance, come in 300V and 600V ratings.

Many 450VAC capacitors are used on the secondary side of a transformer (lighting ballast) where the voltage is 400+VAC. While the supply voltage is from 120 to 277VAC a 370VAC capacitor is not adequate. While in this situation a 370 VAC capacitor may work, it is really not adequate protection for the real voltage incurred. I would use caution when the substitution is not up to OEM specs.