I think Wards can do what you want.

http://www.wardsclassiccarradiorepair.com/

I didn't have a stock radio to start with, that's why I went the route I did. If you have the stock radio, I think converting that one is the way to go. I also found a speaker for my one speaker dash that had left and right tweeters installed in it so it fit in the stock one speaker space. I forget where I bought that though. I put some decent 6x9s under the slotted package tray for the rears and mine sounds pretty good.