I have had a chronic leak at the filler tube on my 66 C4 for 2 years now. Tried different diameter o-rings, RTV, still would drip.
I purchased a repair kit off of ebay for $9.00 figuring it can't be any worse then what it leaks now.
http://www.ebay.com/itm/151507453118?_t ... EBIDX%3AIT
The kit works awesome, easy to install and not a drop on the floor.
I drained about 1 quart of fluid from the pan, my pan has a drain plug so it's an easy task.
Pulled out the filler tube.

Cut off 1-1/2" of the tube with a pipe cutter

Apply a thin layer of JB Weld to the tube and slide on the sleeve

A thin coat of Ultra Black sealer and install the seal into the C4, install tube