i would say 3.8 is good alround ratio av tried them all before goind mtx etc,i would source gear kit from later box and change internals,using later gearkit uses 3 piece molybedlm synchros to,for better precise shift.if in doubt count teeth on mainshaft then teeth on crownwheel then devide the to and= final drive.would advise to have matching mainshaft and crownwheel,otherwise it will be noisy.hope helps.