O2 sensor location

I have a '86 F150 w/5.0 engine it has passed smog with really good numbers year after year. It has been sitting for a year and not used other than to move it around the yard or start up and charge the battery. Last week took in for smog check and it passed the 15mph test, but would not pass the 25mph test. CO2 levels to high.
In reality all levels much higher than last two tests from the last five years. Have only put less than 1500 miles on the truck in two years.
Put fresh top teir (Shell) gas in truck .
Pulled the plugs
Checked and cleaned rotor and cap
Removed throttle body and air bypass valve, cleaned all parts
Took truck for 50 mile cruise on the freeway, high speeds, steep grades, got it good and warm. STILL won't pass. tech said to test O2 sensor and I can't find one on this truck, anywhere! No loose or unused wires ether.
Any help would be would great. Thanks
I have a set of Ford service manuals for 1989 that someone gave me. So far this is only the second thing I have found that differs in the two years. The other was the shape and placement of the air filter box.
thanks again










