PJ, I know you didn't address all your questions to me, but I'm going to give you my opinion anyway.
The block stamping is right above the oil pan below the injector pump. Pretty easy to locate. The area and number will be about the size of a business card.
The K&N's big "thing" is that it allows more air to flow through. In order for it to flow more air, it will also allow mort dirt. Also, MOST folks that use a wet filter, be it a K&N, AFE, etc. , do not keep them properly cleaned and oiled. The bottom line is that you are not gaining anything over a stock paper air filter, so why take the chance of poor filtration?
As to the injectors, I don't think you will notice any appreciable decrease in noise from aftermarket injectors. That said, if you are doing any towing at all, I would not go more than something like a 50HP injector.