I'll appologise up front, I've not read the full thread so perhaps I am pointing out what others already have.
I can't even see youtube at the moment, so I'll go on what has been writen on this last page.
You have a lean issue at idle, 18:1 apparently, and then 12 to14 elsewhere under load. To check plugs the engine must be cut during the condition you wish to see...ie full load engine plug reading....nail it then cut the engine and remove, otherwise you simply see the idle condition with a bit of a mix of everything else. That said, looks to me like your plugs show carbon build up at the base of the thread (running rich), with the tip displaying somewhat leanish conditions. As your meter suggests.
The fact that you have an A/F ratio meter really means that all you do when you check the plugs is confirm the meter probably works.
So, it's lean at idle, and that will tend to give high rpm. Have you checked the air idle screws? Do they have good seats? Are they even? (Count the turns as a starting point). They sound like they are wound out to far (maybe). Are the butterfly's all even, are they set to factory specs on angle...hell, are they a matched set and unbasterdised, or are they bitsa carbs that have not been pulled down and inspected? Of course, check for vaccuum leaks.
12 to 14 is pretty ordinary fueling for NA....well the 12 is not that great, waste of fuel really and will foul any plug given time...no point using long life iridium plugs in that scenario (not that you are but but I noted they were suggested....they were designed to increase service life on hard to get plugs in modern cars....you'll foul them well before then). 12.5:1 is as rish as you'd really want to go on turbo, and that's because the fuel at that ratio helps cool the charge and stay of some deontation under higher boost. On N.A, I would have expected high 13's as a good place to start. On that basis perhaps you can drop one jet size and repeat, see if you stay shy of 14.5 up top but lift the low down AF to something more paletable? Anyway, there are many out there that can tune a carbie better than me, so take it with a grain of salt. I ran 48's on a NORD 2ltr and found I was able to get AF between 13 and 14:1 (I think, I'll dig out the dyno run and confirm, it was always a compromise though, as these things are). I vaguely recall I was running 195 jets?...will also check.
Sorry if the above has already been said.