Although not flathead related, I can answer your question somewhat.
If you 700R4 has a mechanical speedo output, I believe Autozone will make up a cable with the proper ends for you. Make sure you give yourself enough length, and have your original cable with you so they can check out the speedo end.
If it's an electronic speedo output, check in with the Corvette forums as there's a somewhat pricey adapter to convert the electronic output to mechanical.
You can also have a ****** shop change the elsectronic taishaft to a mechanical - also procey.
To me, after all is said and done, I would go with a cheap GPS, attach it to your windshield and be done with it. Both my avatar and my 40 coupe do not have working speedometers - I just switch my GPS unit from vehicle to vehicle as I need it.
Folks on this forum are really good people but they are flathead folks, period. If you have a stocker with a flattie, please, by all means, lurk, post, or whatever but stay connected on this site for some of the best flathead information you will find anywhere. I myself do not have any really "stock" flatheads nor are any of my vehicles stockers - so, I limit my posts and contribute with flathead related issues.
The written word does little to convey someones true personality - hang around and I know you will enjoy it!