You don't have to replace the reverb pot with the R2 pot. Not sure where you got that idea, unless it's from people who decide they're not going to use the reverb and choose to disable it to do the R2 mod. My Mark III has no reverb so I plan on using the hole where that pot would have gone.
The thing is, if your Mark III has reverb and you want to keep it and do the R2 mod as well, there's no place to put the new pot without drilling a new hole.