Add water after fermentation?

I brewed 6 gallons of a Sam Smith Nut Brown Ale clone. Due to some tinkering with my mashing process, I got a whole lot more efficiency than I expected. My carboy is only 6.5, so I couldn't add water at the time, and didn't have a spare one to split the batch.

I was aiming for an OG of 1.057, but got 1.070. By my calculations, I'd have to add 1.4 gallons of water to correct. I'm thinking I could boil that volume with priming sugar at bottling time and add it at that point.

Any reason not to?

Taste the beer first. If it tastes too strong or you feel it could benefit from dilution, add water the way you suggest. If it tastes great, don't change a thing!

I've done this to lower a high FG, but not with that much water. The only thing I can add to what Yuri says is to remember it will dilute hop IBUs/flavor as well (you can calculate the IBU change with your same method). I wouldn't focus on OG in considering this. Look at FG and ABV with the same calculation you did for your OG water addition and see how the results compare to your intended outcome for the beer. If you do decide to do it, you could consider compromising with a less water.