As they were made both ways, you don't have to have custom parts made, you just have to source the parts needed, and do the labor, no kits that I know of.
Unlike a IIa, w/ a III or a Def, you'll have to change the dash to get the steering wheel to the other side. The steering box needs to change, and the front axle. Then, there's the linkage for the throttle. You'll need to flip the heater to the other side (I don't know if you'd need a different heater or not, but I suspect so). The brake and clutch should move over easier, then run your lines.
FWIW, I'd keep it RHD....
-L
'72 SIII SW 88"
'60 SII 88" RHD