Can you respond with rat chaser after the complete resolution of A Years difference, or do you have to respond with rat chaser before you add cards to the top of your deck?

i.e. can you years difference, draw up to hand size, place cards on top of your deck until the # of cards in hand is equal to handsize, then respond with rat chaser to draw many cards.

it happens right after the draw and before you put cards back, so you get to choose what you do for a while, but still dont have that big a hand

Stamp for ziepnir. You respond with Rat Chaser at the moment of its trigger, interrupting anything else temporarily to do so.