Where does the timing need to be on a 89 350TPI for the best performance?

 

It's set at 6* before top dead center stock. Generally 8-12 is a good figure. But advancing it too far is useless because you will only have the knock sensor back the advance down if you start getting a knock. I set mine to 10*. Make sure when you check it with the timing light, you remove the EST wire from the wire leading to the coil. It should be a brown wire rigged to an inline terminal.

 

I'd try 10 degrees before TDC and see if it works well, maybe try 12 also. Make sure to unplug the brown wire along the firewall before checking.