Your desire to have ‘race ready’ unit even if it is not destined to the track is legitim. Consider this: On GT Am Autodelta team cars, auto-lock was ratchet type, with high closure ratio (47% and more on demand), 15 splines and its ref. number was 10533.17.043.E02 (the same was used also on 33 cars so capable to support and manage any power that GT Am engine could produce), and to be mounted on GT Am, the ring and pinion 10551.17.021.99 was requested. This slip diff. was suitable only for the track and not street use, it is not easy to find, and its price is high. In my opinion, you should search ‘standard’ ZF unit used with 2000 engines (it supports perfectly GT Am power as well-is the ‘small one’ as you call the one present on your car the same thing?). Its ref number is 10544.17.043.01 (10516.17.043.01 will do also) and it is 25 splines and ‘mild’ closure (25%) so suitable for street use. If you need further info, send me e-mail.