Nothing is lost loosening the underneath nut, or removing the lines to pull the whole handle assembly if that doesn't work.

Or jumping up and down cussing and threatening it with a sledgehammer if it refuses to cooperate! :D

Thank you all for the valuable knowledge. I tried to remove the nut, but was unable with my tools due to the tight space. The smallest wrench I have that fits the nut is 12 inches long. I also tried a telescoping wrench, but the arm also does not fit between the nut and the sink. I think I have found a wrench on Amazon that should do the trick (Channellock 6WCB WideAzz Adjustable Wrench with Code Blue Grips, Over 1-1/2-Inch Opening 6-Inch Overall Length - - Amazon.com). When I get it in a couple of days I will report back. Thanks again

You need Basin Wrench to take that nut out. If it is frozen, you can take long flat screwdriver ( or narrow chisel ) and gently tap it sideways with hammer in CCW direction. Once you break the grip, you can use other tools to unscrew it from the stem.

I suggest purchasing a set of tub and shower wrenches.
Disconnect the supply tube and the flexible hose to the spout.
Select the correct size, slide it up the supply tube and flexible hose to the nut to insure you have the correct size.
Note that the wrenches are stackable, that is, one will fit inside of another.
Put two or three of these together and you will have a socket that will slide up over the supply tube and the flexible hose, will precisely fit the nut and will extend down below the sink.
Put the handle or a #2 screw driver through the holes and turn. If push comes to shove, you can even put a wrench on the socket to turn.

Works so much better than a basin wrench. I hate basin wrenches.

I go with Milo's suggestion to buy a basin wrench it is cheap and works better then the tub and shower wrenches, see the image below, Thanks.

John

OK so I have finally removed the valve body from the sink. It took about a week of soaking with penetrating lubricant before I was able to remove the nut with a tub wrench. And now to my next issue...

imgur: the simple image sharer
imgur: the simple image sharer

I cannot get the valve cartridge out of the valve body. I have sprayed it with penetrating lubricant a few times and have it in a vice and can still not get it loose with a properly sized socket wrench. Any advice?

Thanks

Also here is the diagram I found for the faucet:
http://www.faucet.com/mediabase/specifications/kohler/561276/kohler-devonshire-lavatory-faucet-k-394-4-installation.pdf

Yes, those stem are certainly chewed up. There is an "O" ring inside that obviously hardened preventing you form unscrewing the stem from its body. I suggest you take large (large) channel locks, grip it well, hold tight ( don't let go ) and turn with your whole body. It should go. If not, start thinking about purchasing new stem.

Back to you

Milo

Don't like the way you have the valve mounted in the vise. If you tighten the vise tight enough to hold the valve while you really pull on the stem nut you will probably distort the valve.

Suggest you take the two mounting nuts (from underneath the sink), screw both of them on to the valve body. Then tighten one up against the other really, really tight. Then put the valve in the vise, gripping the nuts.