resize the canvas for each image to 8X the width, but change the X offset for each one in incements of 300, coordinating that with wherever the image belongs in the sequence. for example:
tile 1: 2400X300, x offset 0
tile 2: 2400X300, xoffset 300
tile3: 2400X300, x offset 600
The white space created in the enlarged canvas will have to be removed (made transparent) before merging layers.
Of course if it were me, I'd just resize canvas once, then paste the other tiles in as a new layer, move into place (zoom in to get it exact), then merge. Voila!