Wood plank flooring gaps?

I have filled "wood floor gaps" in the past with success and some not so(stain matches lol). However this new customer seems to have some pretty big gaps (due to an old house expansion etc.). I am a little bewildered on how to handle this,since they seem to be very finicky folks.
I figure gaps up to 1/8 thk., wood filler with stain to match will blend in and can be camouflaged pretty well. But I am a little more concerned on the ones that 1/4-1/2 how do you handle this? I know its not oak flooring. I believe there is two scenarios one room is old wood planking 4-6 inches wide (knotty pine) and I think the other is more in the lines of a pergo type product planking. I do recall the stain/color on both floors is in the very dark family.
Sorry I couldn't be more specific, will try and be more accurate when I go inspect a second time with pix and take care of some easier tasks.
I am going to ask if they have some spare pieces in the house. I have in the past ripped some oak floors into tiny strips and fitted with some success.

 

I'd be inclined to rip wood to fit the larger cracks and then fill the rest. Even if it's not the same species of wood it will look better than the gap or too much filler.