I've got a set of flow-tech headers. The flange that bolts to the heads are oval. i know they should be square but the oval is bigger than the diameter of the exhaust port on the heads. I know these will fit and work but should i use them? will there be and power loss or gain with these compaired to a square flange? thanks

 

As long as the opening on the header flange is bigger than the ports of the head, it will be fine. You won't lose or gain anything. Now, if the header flanges were smaller than the head ports, you would be restricting/obstructing flow.