my 74 duster had a factory 8 1/4 i went to an 8 3/4 and used my factory driveshaft. no issues at all. after that i went to a ford 8.8 and the driveshaft length was good for that too, but i either had to put the ford driveshaft end on my driveshaft or get a conversion joint, i opted to just stick the ford end on, not change in length.