[Ed Wright]

As Chad said, put an extension on the collectors for idle & part throttle. Unless you have had eight wide bands in the primary tubes to verify all eight cylinders are equal (they never are unless it has been put into sequential mode for cylinder-to-cylinder tuning) putting one upstream may or may not be accurate. Ten percent variation between cylinders is not unusual. At WOT, using one upstream, your indicated 13-1 a/f could be 12-1 or 14-1 on most other cylinders. EGT is not an accurate indicator either. Things other that air/fuel effect EGTs. Most agree that the average you see in the collector is best indicator if individual cylinder-to -cylinder tuning has not been done. Don't tie yourself into some magic number, find the air/fuel ratio makes your car run the best MPH numbers. Don't worry about what somebody else's car likes. Cars I have worked with like somewhere between 12.5-1 and 13.25-1 @ WOT. Mine likes about 14-1 idling & on the return road, with extensions on the headers, it's always in open loop.
Me tuning extensions are 3' long, with a 45 deg bend in the middle.
Your results might vary. LOL