Hi David. Im not sure about a flat head V8 Ford engine, but about 5 years ago I installed a block heater on an older model diesel MF tractor. I found an online catalog that listed all the specs for various block heaters for many different automotive and industrial engines. I cant remember for sure, but this might be it the catalog I used:
https://phillipsandtemro.com/wp-cont...rt-Catalog.pdf
If you know the diameter of the frost plug and the amount of clearance inside the water jacket, you can probably spec one out from the listings yourself.
The wattage seems to range from 400 watts to 1500 watts depending on the engine. You can compare what is spec'd for new and old engines in the listings for each manufacturer (go down to the agricultural/industrial listings to see the wattage ratings).
Hope that helps!