Stack affects reach very little because headtubes are not perfectly vertical. But, the angle is slight enough that the trigonometry calculation on the angle means you are multiplying the reach by 0.995 or 1.005 when you raise or lower the stack by normal amounts. Those numbers are made up, but just to illustrate how minor it is. So dropping the bars won't affect reach. That's reducing stack, which reduces hip angle.
Set the stack the same as the other bike first, then measure the reach on the new bike, then buy the right length stem to attain same reach as old bike. You probably were able to fiddle with the saddle height and setbacks well enough to get those same for same to the old bike.