That depends on what you want to use it for... If you've got a stock 258 l6 or even a 304 V8 it should hold up fine. The TF999 is a medium duty tranny and a rebuild kit isn't very expensive if you intend to do the rebuild yourself.
If however you've got anything bigger than the 304 like a 360, it will probably hold up for quite a while providing the tranny is in a good condition or a rebuilt one, but in this case you'd be better off with a TF727 which is the heavy duty version. This tranny will also fit to the 258, but you would need a 4" lift to clear the front drive shaft from hitting the tranny's oil pan.
Hope this helps.
'83 CJ-7 in Land Rover Country