That was a consideration when I recommended the two NGKs, as well as resistance, albeit not mentioned in the post. Most of the plugs I found that would physically fit were in an acceptable range, so it was easy to overlook mentioning it at the time. The stock plug's rating is 9, the 53599 is also 9, the 91909 is 8 in the event you needed one step colder. It's definitely worth mentioning though, since it might not be known/on the minds of everyone who is looking for a different plug.
So that 95399 should be the right specs.
You run this plug for any miles yet?
They look the same size, or well close the 95399 looks about 5 mm longer on the electrode and contact arm and I would not think the clearances are that tight on the 400 on top of that piston at TDC