I wouldn't worry about it a bunch. You might consider a different rectifier (5Y3) to lower B+ a bit and give you a little more cushion. When those amps were made line voltage was lower. It won't help filament voltage (I'm betting your filament voltage is a little high, too), but unless it's ridiculously high, I wouldn't sweat it.
This might help if you choose to go that route.
Rectifier Tube Voltage Drop Chart - 300guitars.com
Edit: Be aware that some of the alternative rectifiers draw more filament current, but it's not normally an issue with the old Fenders. They have pretty hefty PT's.