This is a good move and should be the first thing you do.
You'll find it much, much easier to blend a greyscale image than a colour one....you've only 256 shades of grey to deal with!
This is also good. You should get a good enough result in a reasonable time, so much so that if you didn't tell anyone they wouldn't notice.
You won't have to worry about colours if it's greyscale.
Here I did some of it using a levels adjustment for one area and a black 'Colour Overlay' layer style for another...
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There are many ways you can adjust the image when its a greyscale, just use whichever suits the area you are working on.
Of course if you look closely you will see the edges but of course you have the advantage of knowing where they 'were' and that there was anything different in the first place.
It sounds as if you already do this but just do one small 'chunk' at a time...you can start bigger but one adjustment may not be correct for all the area, (see the top right adjustment...just not quite right for all of it)...so use masks as well...anything to make it easier. Your larger image will also be easier to work with.
After each 'area' stamp the visible layers and then start again with another area. Before long you'll be done in no time.
Regards.
MrToM.
