Hang on, copy and paste is definitely not my recommended technique.  Never C/P, only Place.
 
 
	Blend If can be awesome, but it's not infallible.  Make sure you always check its results VERY carefully.  I suggest putting a bright red layer behind your Blend-If-ed layer to help examining for flaws.
 
 
	Well, I'm pretty sure if I was doing it I'd want to do the one-image-at-a-time method.  My computer is no slouch, but it definitely doesn't have enough power to be placing an overabundance of full-res images at once.
 
	But it's up to you.