The 850-T has a unique clutch cable (1709 3002) with detachable upper barrel end, there was only ever one length as far as I know. The complete clutch lever assembly is unique to that model as well. If you want a longer cable, find the more common Convert/T3/G/SP lever assembly and a longer cable from one of those models as well.
Alternatively, have a custom cable made (Barnett can do this) to work with the original clutch lever assembly.