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If your U joint or flange bolts to the tail shaft you will need a telescopic driveshaft as seen in the pics of my 67 Camaro. Some 4L80 transmissions use a bolt on U joint. If your tail shaft has a threaded hole in it, it uses a bolt on U joint.
If you are running a truck transmission you will need to put in your slip yoke to U joint measurement. See diagram below.