There is no such thing as an original overflow tank on any 65-70 Mustang.  they did not use them. I am
not using one now, and my radiator does not lose any water.

When I hear of someone having overflow issues such as yours, it is usually because they are overfilling the radiator.
There should be 1” between the top of the coolant and the bottom of the filler neck. The cap should be 13#.
Overflilling will result in the radiator cap letting out fluid until it reaches the proper level

You can add a coolant recovery tank and correct style cap that works with a coolant overflow recovery tank, but it is just adding more parts to a car that are not needed.

if you race the car, most sanctioning bodies do require a coolant recovery tank, but that is a totally different ball game.

A 66 didn't have an overflow tank originally. Is it using the windscreen washer bag as an overflow tank? My 65 was setup that way ... doesn't work.
 

Last edited by 50vert (8/10/2023 8:51 PM)

I have a small cylindrical chrome tank installed right beside my radiator. My thinking is if my car overheats and starts spewing coolant out it has a container to catch it. If it spewed out the normal rubber tube, that hot coolant would definately stain the underside of my car.

I installed a SS overflow tank similar to the one below, more for my own peace of mind. 
The purpose of the overflow tank is to allow more coolant in the radiator.  You fill the radiator to the top, leaving an air gap at the top of in the overflow tank.  This system allows the coolant to escape the radiator as it expands as it heats up to operating temperature, and keeps the radiator full.  As the engine cools, coolant is drawn back into the radiator keeping it full. 
Note that one may not need an overflow tank if they aren’t having any cooling issues.  If one doesn’t have an overflow tank, be sure to leave an air space in the radiator for the coolant to expand as the engine heats up.
If you are blowing coolant out the overflow, you may have filled the tank too much.  If you’re not having cooling issues, try leaving a larger air gap at the top of the overflow tank.  
Also note the radiator cap needs to working correctly, and have the correct pressure rating.

https://www.amazon.com/Overflow-Reservoir-Stainless-Universal-Accessories/dp/B092ZWRGP6