Well the piston swap was going great. Got the engine out, head cover is off, and finally just had the two under head nuts to remove.

The rear nut rounded off It's such a small space, I couldn't get any of my sockets in, and using a 12pt wrench rounded it.

I've tried heat, some vise grips (needle nose, all I've got) At this point I'm at a loss. How do I get this nut off. Its stopped my whole project.

Any thoughts/advice?

Start out with soaking the nut and stud with penetrating oil.

As I posted on FaceBook you need a Craftsman 6 point Craftsman box end combination wrench.

You may have to take a wooden dowel and hammer to tap the wrench into place and maybe put something on the end of the wrench to get more leverage.

Make sure that you are turning the wrench the right direction.

Kenny G

I was hoping to avoid buying new tools, but looks like I'l have to.

I wouldn't know if the vise grips fit until they arrive :/

I'll try a 6 point and I'll get some PB blaster, but the edges are getting pretty mangled.

If I can't wrangle it off, what are my options? the screw portion seems to go through and up to the intake.

Also, how much do I need to worry about hammering into the thing. Do I risk cracking portions of the block?

You might take a small fine file ,use it two clean up the flats ,then try a six point box end after soaking it a few times (or days) with penetrating oil........avoid buying tools? tools are like uncut diamonds,and the proper tool is a cut and polished diamond. If you buy them as you need,they shouldn't cost you much over time, and will keep you from problems like this in the future.  I would try not to damage the stud going up in the head as it is steel and the head alum. and is difficult to remove and replace ,the nut isn't.  you may want to replace and tighten the nut on the front of the head first ,before trying to loosen this one, the head may have "cocked " (lifted in front) allowing the rear to bind.this is after all just one of six head bolts and should have been only torqued to about 20 ft/lbs.(you should have loosened all of them a little at a time,in stages, not one at a time completely,that's how their put on ,it keeps the head flat, and the head gasket evenly compressed ,not so now!For that matter you could replace all the head bolts and try removing this one first!THEN the front and the four on top evenly in stages.   I would try this first, you have nothing to loose and it shouldn't cause any damage if you use a torque wrench ,don't exceed the proper torque or you could damage threads or stretch the studs .

Out of curiosity, what brand of wrench was it?

There are no rules when fighting a problem. Tools are just tools. You can get a junky wrench at a pawn shop, clean up the area you want stuff to stick to, SURGICALLY apply a light grease to everything else, wipe a bit of epoxy on the nut and inside the wrench , wait, break it loose. I wouldn't use JB Weld,, but something you Can break loose, like a five minute epoxy. I used Devcon, because it's good. For this, I'd probably use Duro, because it's not that great.
Super glue gel might be a choice, since it will dissolve with
Acetone.
I have been known to clip a strip of a thin feeler gauge, hold it in position with needle nose pliers until the wrench has it held in place and tapped the wrench down into position.
If you can get a pin punch and shape it up to drive the smeared part of the nut back home, that might help..

The thin, runny superglue Might work,
Even pounding a small, Hard wire as flat as you can might give the wrench the bite it needs.

I was changing wheel bearings on a friend's car. My socket was sloppy. Option, go to town, spend twenty bucks or cut up a feeler gauge and shim it... We didn't go to town. He was Very skeptical and really didn't want me to do that. It worked and left no rounded edges on either nut,

Indeed PB Blaster is the best I have ever used !

justin_o_guy2 wrote on 02/10/17 at 15:52:21:

the ATF/acetone doesn't spray and it's a hassle.

Put it in a common spray bottle. ATF/Acetone is my go-to penetrate