So, you have your's in a vertical frame now? If the Galaga already has a 4600 in it, then you can just swap everything over to the other frame.
otherwise you should track down a K4600 horizontal frame (not hard to find) so you'll have all the mounting stuff in the right place.
otherwise you'll have to drill holes for the plastic chassis frame pieces for a standoff, figure out where to mount the flyback, etc. Also, some 4600 frames have the degaussing coil mounted in a shield behind the tube rather than around the tube itself, so you'll have to move that, too.
Now, if you want to get creative, you could get a 48" U-shaped shelf bracket from Home Depot, cut two pieces from it to attach to the mounting tabs on the vertical frame (long enough to reach out a little past the length of the tube on each end), attach THOSE brackets to the brackets that go on the horizontal frame, and then slid it in....