the 02m has been proven to take 1000hp/900ftlb in the states with a few simple mods/ improvents, mainly selector forks and diff bolts
the jc5 will hold at best 350ftlb -400ftlb max, depending on condition and how the transmission is being used, also your looking at a twin plate clutch once you see over the 350ftlb mark, they are not cheap,and dont last long either, where the 02m o.e clutch will just about take 350 ftlb in standard form, and there are single plate clutches out there that will take almost 600ftlb for sensible money
as regards cost, it really does depend on how much hp/ftlb you want to run, but now you know the limits of the jc5 type box
to convert to 02m your looking around £2000 minium , depending on how cheap you source a box , fabricate brackets +driveshafts, clutch and bearing,flywheel , selector system, but long term i think its still better value than pulling apart jc5's every month of the year