I agree that a T5 would be a great addition. However, an original type overdrive (Muncie SM319 w/BW OD) would be a drop in for your 60 without any modifications. Electrical components are required as part of the system. They are still available and reasonably priced.
Now that everyone knows about the 4:10 option for the "corporate" rear end they are becoming more scarce and of course more expensive. And remember these are used parts and may require rebuild. It would be wise to consider all options.
Fred
