Hi Kurt,
Here's a few pics of the 1376973 part # cap that I believe is correct for 69 Chevelle. I have a few variations regarding the clocking of the lever handle that maybe somebody more versed in 69's can explain but it's definitely a deeper shoulder on the retaining tabs than the vented versions have which should explain why you can't get the vented cap to seal on a 69 tank.
Verne, can't say for sure positively on the El Camino application except that all my reference material 68-69 printing Chevrolet parts catalogs all call out for a vented cap on El Camino and station wagon.
Chris
