Do Shorty headers fit factory exhaust?

I want to preface this post with the fact that I'm 17 and basically clueless about the hobby, but I'm trying to learn.
I have a cracked manifold on my 68 valiant's 318. I'm wondering if you guys would know if the headers I found that jegs is selling could fit my car with no exhaust modifications. Basically, I was impatient and bought a header back system from summit, and then paid a shop to connect the difference from the factory manifolds. Since I spent a bunch of money on that, I don't want to go to long tube headers because it would've been a waste of money and I could've bought headers from the beginning. I also figure that if I'm gonna mess with the manifolds, it wouldn't hurt to do a bit of improving, so I got onto the idea of shorty headers, but the only ones I could find to fit an LA were ones labeled for a Dakota with a magnum. Supposedly these bolt in right in place of the manifolds on the truck, but is that the same as my 68? I've heard of using magnum manifolds on non magnum LA motors to get more flow, but do you have to do any exhaust modifying to do that?
Headers I found: http://www.jegs.com/i/Flowtech/387/...B2jAwet80Ta7zgVp_GGhKoK-TlpQpCCcvgaAkat8P8HAQ

Also my 318 is from a 69 cuda and to my knowledge is bone stock, although I want to build it up which is why I think headers are a good idea cause it gives me growing room.
Any help is useful!