I'm not sure if you are looking for a factory overflow tank (I've never seen one) or one that looks factory and fits our trucks. - If you are looking for an overflow tank looks good & fits our trucks you might look at the ones for the Ford Trucks. They are a Radiator Over Flow/Windshield washer combo and the cut of the tank fits the angle of our inner fenders almost perfectly and as a plus you can now add windshield squirters at a later date if you want. This is the set up I went with on my 64 C-10. I moved the horn relay and voltage regulator to the other side of radiator support using the original mounting holes and grommets for both. I later though did go to a one wire 120 amp alternator. I got mine from the wrecking yard BUT you can get it brand new from Rock Auto for $28.00 if you prefer to go that route.
Rock Auto Overflow
http://www.rockauto.com/en/catalog/f...ery+tank,10457