78.1mm I believe it is. As long as you go bigger, you can purchase rings that will bring the size down so you maintain hub centric spacing as opposed to lug centric.

 

Yep but some places I see 77.8 & others I see say 78.1 The viper replicas from factory reproductions are 77.8

 

So I would be safe with a wheel with a 78.1 bore? Or do I NEED to go a little larger?

 

77.8mm is the actual size of the hub. 78.1mm is a typical hub bore dimension as it will give a little more clearance to slip over.

You either need wheels with the proper hub bore or hub rings if the wheel has a larger bore to keep the wheel hub centric.

 

Anyone know where a reputable place to buy these hub rings from? Are they really necessary?

 

Motorsport Tech would be a good place to start. But there are lots of manufacturers that make them.

They're necessary if you don't want wobbling issues at highway speed or the risk of snapping a wheel stud off.