Well, firstly, that slanty shouldnt need one. If the pump and fan are working, it should be very difficult to overheat. And the overflow wont change an overheating issue one bit. It will just catch the mess if you overfill the rad.
If you have A/C the temp may come up some.
If the water temp gets up over a true200*F at over 30 mph, then you should suspect a cooling system problem; such as restricted rad cores, a bad pump, collapsing lower hose, etc.
If it gets over a true200*F at under 30mph,suspect a bad pump, or a slipping belt.
The above assumes; 1) the stat is opening and staying open, and 2) the ignition timing is not retarded, and 3) the carb is properly calibrated, and 4) the tires aint flat,brakes aint dragging and you're not towing a humongous trailer behind it!
But to answer your question; If you cant find a good system in the local wrecking yard,heres what you can do; Find a plastic container with a screw top. the bottle should be about 2 liters capacity, oops 2 qts. It should be about 2 to 3 times as tall as it is wide. The screw top needs to have room for a rubber hose, and a vent hole.You dont really need the screw-top but its handy to keep the hose up off the bottom. Then find a hose that fits on the little nipple located on the rad,just under the cap. Run that hose over to wherever you have mounted that bottle,(usually on the passenger side of the rad and sitting on the frame),and through a hole in the cap, and down to about 1 inch from the bottom. Cut the hose off at a 45* angle. Poke a second hole in the screw cap, about 1/8" to serve as a vent. Finally fill the rad right to the top, and put just enough coolant in the bottle to cover the bottom of the overflow hose to a depth of about 2 inches from the bottom. When the engine is fully warmed up it will(if the cap is good) puke its coolant into the bottle. when it has finished puking, mark the bottle with a sharpie, and label it HOT-minimum.Then fill the bottle to within 2 to 3 inches of the top, and mark the liquid line "HOT MAX". URdone.When the engine cools off it will(if the cap is good) suck the coolant back into the rad.
If your rad does not have a nipple on the filler-neck, then you're screwed. lol. No no, you're not. You just cant run a vented bottle. You will have to run an expansion tank. Oh wait you already have one, in the top 1inch of your factory rad.
So then you will need to find a pressure vessel,capable of withstanding the coolant temp for extended periods of time, and capable of withstanding whatever pressure your cap is rated to, plus a safety factor of 2 or 3.So if you're planning on a 13 psi cap, you will need 13 x 3 =39 psi rated vessel.
This vessel will be plumbed a little differently. It will now be an expansion tank. It will need to be located somewhere under the hood as high as possible.Its cap will need to be a little higher than the top of the rad-tank.Then from the bottom of the tank you will run a hose just about anywhere into the cooling system, trying to keep the Tee-in place below the bottom of the tank.After that its same as before; fill the rad to the top, put a few ounces in the vessel and mark it"COLD MINIMUM" ,leave the cap off, start her up, get her hot. When the coolant level stops rising in the vessel,fill it up to about 2 cups short of full,make a line at the liquid level,mark the line "HOT FULL",and replace the cap.URdone.
NEVER open that cap unless the coolant is cold.
If you cannot mount it that high, all is not lost. Mount it wherever you can ,on temporary mounts. Remove it to a higher location until it is filled, as previously described. Tighten the cap, and remount it. Now, never remove that cap as before, but also do not remove the cap, while said cap is below the top of the rad. Why? Guess...... Thats right; water always flows downhill.
Okay, so good luck
