Gas cap mix up today!

[70c10]

Today I went down my dad's to work on his 70'. I bought him a new gas cap at Carlisle over the weekend but didn't realize it was apparently for a 71/72 truck. After putting it on the truck it wouldn't idle at all. I had to really rev it up to keep it running. It didn't dawn on me that the 71's started using the gas vent setup. I noticed when I removed the cap it would let out a huge vacuum sound!. Took me an hour of checking things and getting frustrated. I put the old nasty cap back on and it roared to life with no hesitation and idles like glass again Just thought I would pass it on in case you run into the same thing The main reason I went down in the first place to figure out why the headlights stopped working. Turns out both headlights low beams are burned out at the same time