c-15 was just one of the model numbers for narrow gauges...eg...b-12...was another.....my gear ratio was 3;55...lots of power...and top end...the c=6 is fords heavy duty auto...for the 300 /six....and if you can find a c=4 auto....they work just as well,and are a fair bit smaller for installation....had not been on here 4 a long while......hope this is still of use!!!
kpm