Yeah, those big 'ol 12 holers can be real foolers, run real nice and smooth and powerful. Until, as you experienced, they just puke right out on yah, not much warning that they're getting "sick".
You can drop a whole bank and not really notice it all that much, runs pretty good on one bank. That's probably what has been going on, and now the "good" bank wanted some attention as well. ONE of the banks has to run! LOL!
It's real nice how they have that engine set up as far as getting to the plugs. Hardest part of the whole job is carrying all those plugs to the engine. Getting used to it though with these twin plug engines.