The wiring will be OK as long as it's in good condition, that would be the least of your concerns. Are you looking for easier starts? It would really be better to do a full 12V conversion especially if the car is driven regularly. Or if you're concerned about originality stick with 6 volts and make sure all your wiring and electrical components are up to snuff.

If you install an 8V battery in a car that has a 6 volt electrical system the voltage regulator will need to be adjusted to put out enough voltage for the battery to charge. You'll burn out lights sooner and your dashboard instruments will not work properly.

There's a topic thread over at Jalopy Journal that discusses the pluses and minuses of going with an 8 volt battery in a 6 volt car:

https://www.jalopyjournal.com/forum/threads/8-volt-battery-in-a-6-volt-system.801300/