I used this one on my 68 charger because I had the exact overheating problem at idle plus my A/C would stop blowing cold at idle. Installing this fan solved both issues.

http://www.the-fan-man.com/shop/size-pus...316f4ddd4b70a22

But you may have to upgrade your alt.

I have it wired to come on automatically with the a/c or if the temp goes over 175.