posi

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hey guys probably a stupid question but im not sure so here goes!! i have 72 cutlass supreme 350 4barrel 350 trans everything stock from factory! want to put posi in ! can i reuse original rearend housing and just put a posi carrier in it or do i have to completly change rearend to a posi unit and if thats the case what can i use that will bolt in without any modifications?? can anyone set me straight!just got the car running to drive on street and i noticed that it doesnt want to shift into 3rd gear looks like someone allready replaced vaccum modulator and i do have a vaccum source from the intake manifold at idle its a steady 18 hg the only way to get it to shift into third is to hold pedal to floor and wait it will shift but at a very high rpm not normal for sure lol!very high milage and lots of things wrong with the car in general so im just trying to straighten things out as they appear. dont want to start dumping big cash into something that might not need it! can anyone throw some idieas my way dont really know a lot about transmissions maybe im over looking something! 1st and second seem normal no slipping but 3rd has a mind of its own!lol thanks for any help anyone can give! p.s. fluid is ok carb needs overhaul wet around accellerator pump! i did notice that kickdown cable connects from gas pedal to trans not to carb like i thought it would????