Yeah, I noticed that with my E-Body as well. With mine, the fluid level does not change much in the overflow...perhaps an inch or so after the initial equalization.

However, then the car is pretty hot and I stop somewhere it still overflows just a little onto the ground. Now that it is cooling off, I expect that behavior to stop.

I question if your using the correct style tank, per my link above. That style tank must be mounted above the radiator and is designed to be used as fill for a radiator. That style won't work correctly down low with our radiators that are designed to be filled through the radiator cap.

I have a 74 and am not worried about originality. I just got a rectangular generic bottle and mounted it up inside the driver side fenderwell. Ran the hose stockish along the radiator top, then down through the radiator support. Nobody has ever noticed that it has an overflow bottle.

""I question if your using the correct style tank, per my link above. That style tank must be mounted above the radiator and is designed to be used as fill for a radiator. That style won't work correctly down low with our radiators that are designed to be filled through the radiator cap.""

I used the Dorman 603-121 which is called a "Pressurized Coolant Reservoir". The tank is mounted lower than the top of the radiator and the fluid moves back and forth with no problems.

After my last drive, the coolant reservoir once again went dry. This is the fourth time that I have added coolant to the reservoir and the radiator sucked it in. I must have had a decent size air bubble in the system.

Thanks for the replies. I am going to keep adding coolant to the reservoir until the level stays within the height of the reservoir after a full driving and cool down cycle.

I too would like to use your method of using the wiper tank for my 70 Cuda. Can you give some detail as to how you did that?
Did you use the Manual wiper tank with nipple at the bottom, or did you have to modify it? I don't know if the overflow hose will fit that nipple. I currently have an electric wiper tank.
My radiator overflow hose points out to driver side, how did you route your hose to the wiper tank?
I totally understand drilling the tank so vacuum will allow coolant to flow back to radiator.
Hope you see this and THANKS.

I picked up a recovery bottle from a 2000 Toyota 4Runner. It is a nice square shape that will fit next to the radiator on my '65 Coronet.

Adding a coolant recovery setup like this is a good deal on the older cars since it allows you to run the radiator full of coolant. The extra coolant adds some thermal capacity to the system.

Trying to keep my car stock looking by using the wiper tank. Just need specific info how its done. Thanks for your solution.

Took a chance and purchased s Repro manual wiper tank. The 5/16 overflow hose will attach to the nipple at the bottom. Just put on a 3/8 spring clamp to secure the hose. My overflow hose exits out towards the driver side. So I purchased a bulk length of 5/16 overflow hose. The hose exits as it normally does, but I had the hose routed below the radiator towards the wiper tank. The overflow hose will lay on the radiator support. Very simple.
I then drilled a hole near the top of the tank that is hidden from view. The hole is just large enough so I could press in overflow hose so if it gets overfilled, it will vent to the ground.
Also purchased a radiator cap that is vented. The overflow did go into the tank and when the engine cooled, the coolant went back in to the radiator.