I had OLD GOLD make a custom one for detroit diesel 61 using a design that I sent him and it turned out perfect! You could have him put the old cummins logo on it, which would look pretty good and fit nice with an older truck.

http://img109.imageshack.us/img109/9025/img0176sm.jpg

Here is a good example of the older cummins logo

http://img401.imageshack.us/img401/6514/cumminslogo.jpg