Getting the Nikki off the engine is not a lot of fun, Back in Ye Olde times i recall seeing a special wrench to do it, and it was hopelessly exotic,


and so what i've always done is just remove the vacuum rack, and then its pretty easy to get in there. unless you need to take the carb off again (and again and again). this last time i had a problem carb, and i ended up making a wrench to make life easier.

you will need a sacrificial 12mm wrench, mine was a new Craftsmen, i think you'd want something cheap new, more chance its a crappy soft metal
a combination wrench is best, a box is too tall for one of the nuts(!), some kind of heat, i used the gas stove, and some kind of way to put leverage on the thing.
i used the space between the concrete pads right outside the kitchen and my foot.
so you heat the wrench, at the spot you want to bend it, and mine didn't change color or really give any clue how hot it was, but there was a temperature where it would bend with great force, and then a little hotter it was like butter

it took a couple tried before i had enough angle. just heat it up, put it in the crack and step on it....

 

Your first pic looks like a 12mm distributor wrench. Your right about needing an open-end for that one nut though.

 

Hopelessly exotic,lol,no. Open end,not necessary,if you have the correct wrench. This one has to be at least 40 years old,seen countless carbs. 12mm on one end and 10mm on other. Was great for valve body bolts in late 80s Datsun cars,remember them before they became Nissan?