Great project. For controlling your overdrive A500, look at this page:
http://www.transmissioncenter.net/SwapInformation.htm

In particular, look at the #727518 for controlling the transmission by way of vacuum. It is labeled for the 727 to A518 swap but it should still work on your A500.
If you get a chance, can you post a picture of the modified transmission mount piece? Dimensions would be nice too. I would love to do the same conversion. I went from a 904 to a 999 to get the same low gearing and a hydraulically controlled lockup. Looking for the same increase in fuel economy.
Thanks,
Craig