I don't know what you mean for the 'common conversion' for a 18RG / W50 (I will assume here).
The standard T series diff in a RA23 is fine for this engine / trans combination.
Take some photo's of the diff and post them here and we can take a guess at what it may be.
To be 100% sure of what you have, slip each axle out, pop off the tailshaft and unbolt the diff. Count the teeth on the pinion and crown wheel, and divide bigger by smaller to get the ratio. Measure the diameter of the crown wheel as well, to get the diff series. That will tell you what you have exactly.
Cheers,
Mitch.