trying to figure out how much flwo overflow has

Ok the 2 holes that are there. No bulk heads or anything one is 1 3/4 inch and the other 1 1/2

 

Looks like you have a 1" overflow and a 3/4" return which is very common.

I get right at 750-800 GPH through my 1" Stockman standpipe overflow and the same back to the 100G display via a Water Blaster HY-5000 pump at just over 4 feet of headloss and fine tuning with a ball valve on the discharge.

Here is a link to my Bulkhead Installation Tips sticky post here on 3Reef you may find useful:
http://www.3reef.com/forums/new-hobby/bulkhead-installation-tips-111422.html

 

So it's safe to say it's most likely 700+gph right.
Now when I do this I put a tube pvc or what but that is about as tall as the over flow gate there annoys right