It is not really possible to switch to a Series II style dash. The bulkhead on a series II had most of the dash welded into it. The small gauge panel bolted to the upper and lower dash rails that were part of the bulkhead. On a Series III the inside surface of the bulkhead is mostly flat and then the upper and lower dash bolt onto it.

If you really want a Series II interior you have two options.
1. Swap out the bulkhead on your Series III with a Series II bulkhead and dash.
2. Sell the Series III and buy a Series II.

The second option is much easier. I could help you locate a good Series II if you are interested in going that route.

Regards,
Mike