What determines internal or external balance

I searched the forum and had no luck. Despite my accident and broke my crank , see engine stand thread below. I was building a stock motor for mock up but I was going to make it run and possible sell it to fund the project since my 501 would come avail when the 580 is complete for the Mustang. Here is the question going forward. when I received the F100 project in boxes and engine was disassembled I had a crank spacer. It was pretty ratty with wear groves all in it however I cant find it. Well I will need a replacement crank so how do I know if the replacement crank will need a internal spacer or external? I have a new flywheel and the original balancer . Thanks

Does the flex plate have a weight on it if it does it is external if it doesn't have a weight it's a internal. Did the spacer have a hatch weight hanging off of it if so it is external if it is just round it is a inturnal. The balancer will work with both internal and external.

Any replacement crankshaft for a 580 will usually not be balanced "out of the box", (unless you pay an extra amount for it to be done).  In that case you will need to provide a bob weight and then have it balanced any way you want to pay for..

Thanks , flywheel has no weight but its unused in the box. spacer was internal balance piece. So what I have learned , they were not a match. Time to buy some parts.