driveshaft length for a833od

Dose anyone know what length of driveshaft would be required for a 70 Duster with a a833od from a "f" body to a stock 8 3/4 rear?, we are putting this factory 4 spd car back together but using a o/d trans instead of the factory 3 spd and it is a 318 car and it didnt come with a driveshaft, thanks.