The link below will get you to herm's info regarding overdrives,
The P.T.O. is usually removed and the overdrive takes it's place - but herm has an adaptor so you can run both the overdrive AND the P.T.O.
The link has a listing of which overdrives suit which particular make and/or model of jeep, there is also instructions for determining how many teeth your transfer case has [which will relate directly to your P.T.O.]
When you nail down which P.T.O. you have - and confirm that your transfer case is a Mahindra, give herm a buzz and see if he can supply the correct cogs to match your P.T.O to your transfer case.
You have me a bit jealous - I have been looking for a P.T.O. rear gearbox for some time, with no luck - and have been tempted to cobble one together using salvaged bits and pieces.
[the rear cross member mounted gearbox is normally a 1:1 ratio - all it does is reverse the direction of rotation.]
I have 2 x Mahindra P.T.O.'s in my shed, tomorrow I will dig them out and count the teeth for you as a reference.
http://hermtheoverdriveguy.com/application-information