the length of the rear shock depens on what you'll do with the front suspension!
but with a standard indian bullet frame, and standard sprockets you'll be limited to 320mm.
or as BW mentioned you will have to fit a automtic chain tensioner, and also a chain guide on the front of the swing arm(or your chain will saw through your swing arm), and a guide wheel on the top of the chain.
as the swing arm flexes up past the mid point(neutral) the automatic chain tensioner stops working and the guide wheel will take over.
and also if you change the rear to more off road/trial(softer) use, you will have to adapt the front also.
or an inbalance will ocure, as the standard hard springs in the front will transmit more to the softer springs in the rear!
upsetting the rear suspension in doing so.
always keeping in mind the weight distribution!
NJB are from what i now in the past, non revisable and made of steel --> hence cheaper