Yeah, I ate one of those springs the other day...

Series plumbing:
From the pipe pressure tap to the vent of one tank. From the clunk of that tank to the vent of the other tank. From the clunk of that tank to the engine. Simple.

Weight differential, like left to right? No, never noticed that at all. The tanks are not far enough from the center of the airplane to matter.
Fore and aft? Yes, most definitely. You will notice a difference as the fuel burns off. But the saddle tanks can be put closer to the CG and you will notice it LESS than the original tank all the way forward.
Just mount them as close to the CG as you can. If I remember corectly, I had them right in front of the fan former, and at carb height, on my F16's. On my F86's, it was right behind the fan former, all the way at the bottom of the fuse.