Oh crap..... Internally balanced engine with an external balance flex plate?!

I would contact the machine shop, more than likely they balanced the rotating assy with the the balancer and flex plate as one unit. If that's the case I would just purchase the same part number flex plate and use it. If they didn't balance as a unit I would use the internal balance flex plate. But the shop is the key in my opinion. But there are others here with a lot more knowledge than me.