I am going to keep adding to this list.

http://www.classicautoradioservice.com/index.html

http://www.radiosforoldcars.com/newradio0F.htm

http://www.wardsclassiccarradiorepair.com/id2.html

http://www.normansclassicautoradio.com/
I think this is the company that refurbished my radio and added FM (and stayed 6v). I'd still call around for quotes and maybe ask for references from a few satisfied customers.

Get a phone estimate. Understand that it is not easy to estimate over the phone, without inspecting/testing the radio. That radio, without the speaker/plate, can be shipped Priority Mail for about $12. If the final after-inspection estimate is too high, pay to get it back (agree in advance on the return-shipping cost).

Private Message me if you want more advice on this.