904 kickdown,need help!!!!

I had bought a kickdown from someone called doctor mopar for my 273 ,i installed it and later on found out it was from a 1965 fury,its never kicked down,seems that on mine the linkage snapps on to my carb,on this other one that i have it has a spring and what looks like to be a slide that goes over my carb housing.Either way i cant seem to get it to kick down and to me it seems that its shifting to soon.2 nd gear around 8miles and 3rd around 20.Any help will be appreciated.