You are going to have problems, since the two images are not in the same proportions (width by height).
With the new image of 400 x 250px open in Photoshop, go to Image>Image Size and enter '300' in the width box. If the 'Constrain Proportions' box is checked, you will see the new height becomes 188px, not the desired 75px. If you don't mind the picture becoming out of proportion, leave the CP box unchecked and you can enter any measurement you like for the height. However, the image will look 'squashed'.
Your other choice is to crop the height down to 75px once you have reduced the width in proportion. This will of course lose some pixels from the image but at least it will not look squashed.