904 transmission

You can use any 904 you want to. they can ALL be upgraded. The A-500 has heave duty parts that readily exchange with the 904. The A-999 is the best 904 to start with, because you have ledd to do initially, but you can upgrade any of them to A-999 specs. Just takes time findin the parts. Here's a great site with awesome info:

https://www.wittrans.com/default.aspx

Open the A-518 in one window and the 904 in the other. Go down the list at how many part numbers are the same. Course, depending on your 904, you may not have the five disc direct drum or the four disc forward drum, but they fit right in any of them and on that site, they are horrifyingly cheap. lol