It would be rare but you could have gotten a DS21 with a Borg Warner auto box. From memory 71/72 was the year they introduced it shortly before bringing out the DS23. If it is a BVH then you don't need to do anything special as the clutch by deafult is engaged. Just mate the motor to the original flywheel or delete the clutch entirely and you're done.
The gearboxes are a bit of a lottery depending on exactly which year and model it came from. For an electric conversion you really want a 70s ID19b/Dspecial gearbox mated to a DS final drive which gives you a 4th gear that is only a tiny bit lower than the factory 5sp box. It's a combination only recommended for EFI cars which have a lot more torque to cope with the higher 4th gear ratio. The Dsuper5 can have a lower 5th gear than a DS 5SP.
The only way to work out which box you have is to read first 2 numbers on the tag rived to the box under where the steering rack would normally be or start counting teeth. This is the one on my '70 DS21 BVH note the "65" code
Harry Martens put this list together of the codes for standard gearboxes
63 : ID 19 B 4 bvm
64 : DS 21 M 4 bvm
65,71 : DS 4 bvh
06 : Dspecial 4 bvm
14 : Dsuper 4 bvm
75 : Ideal 4 bvm (?)
106 : SM 5 bvm
107 : DS, DSuper5, 5 bak, bvm
108 : DSuper optionale 5 bak bvm
And to correlate those to something useful, here is a table of all the factory gearbox configurations for 5 bearing short stroke engines. The table also lists out a few custom combinations.
DS GEARBOX RATIOS
The parts are interchangable between any of these gearboxes so you can mix and match ratios. The only limiting factor is the gears are paired so you have to swap 1st&2nd together or 3rd&4th together.
Note that the 3 bearing long stroke engines have totally different gearboxes.