Making a Pola-Instax 455: Putting an Instax 100 back on a Polaroid model 455.

This details the steps of taking an Instax wide 100 back (see • Preparing an Instax wi... for instructions on how to prepare), and mounting it onto a Polaroid model 455. This could be done with any Polaroid pack film camera with bellows, though it would be best to use a model that has the extra ASA settings (not just color and b&w) since 300 ASA is used with Instax wide film. Yes, Instax film is ISO 800, but I think because these cameras are over 50 years old, the electric eye doesn't work as well and underexposes by around one stop, which works out well at 300 ASA.
The back is removed from the Polaroid by drilling out the rivets holding it on, and the plastic trim on the left side is removed. The bellows is removed by uncrimping it from the back. This is a delicate job and doesn't have to be done, but if you want to move the lens body back, it's much easier to do it with the bellows removed. The reason why I think it's best to move the lens body back is because the rangefinder works through the whole focus range. The lens body is moved back as much as the film plane moves back because of the padding between the Instax back and the original Polaroid film plane.
Cutting is done with a hack saw and dremel, and a file is used to clean up the cuts. This isn't as difficult as it looks. I found the hack saw works best in doing the long straight cuts, and the dremel for smaller cuts. The file works quickly for smoothing the cuts.
The Instax back needs a little modification: the lip near the rollers needs to be cut off, and a notch on the left side to make it fit with the bottom of the Polaroid. Stuff a rag inside the rollers to prevent bits from getting in, otherwise it will need to be cleaned later.
Figure out how much voltage your Polaroid requires, then solder a wire to the appropriate battery terminal to supply that voltage. In this case, since the model 455 needs 3.0 volts, the wire is soldered to the 2nd battery terminal, then led outside the front of the camera along with a negative battery lead.
A piece of 6mm craft foam is used to interface the two cameras. This provides the clearance necessary between the cameras and the light seal. This foam will be compressed slightly to be sure there are no light leaks between the cameras. The craft foam is cut to the size of a Polaroid film pack, then the inside image area is cut after overlaying the instax back and tracing the image area, then cutting it out. You'll notice the image area is oriented to the left and down a bit. This offset will be taken care of when the lens body is moved forward.
A piece of 2mm craft foam is used as a light seal between the rollers and the front of the Instax back. This compresses with the Polaroid body when mounted.
The back is mounted to the Polaroid using a tuna can. Cut some straps and predrill holes for the screws. Predrill the holes in the Polaroid and and mount the strap onto the Polaroid, then while pressing down on the camera a bit, predrill the holes on the back and screw it on while compressed. This will ensure a good light seal. Do this for the three straps holding the back on.
Solder the wires from the back to the camera, install the batteries, and test the shutter of the Polaroid, and the Instax back to see if it ejects the film properly.
The trim took the longest: cut the old back of the Polaroid camera, trying to preserve the hooks in the latch mechanism so that the trim will snap in to place on the back of the Polaroid. In effect, you'll be closing the door of the Polaroid back when snapping it in. You can release it later by operating the door latch on the bottom of the Polaroid.
Other trim: the piece under the flash, the left focus knob, and the trim under it. Use hockey tape to cover over the curved top of the Instax camera, and anywhere else you want to seal out the light. Hockey tape is light tight and looks OK.
The lens body is moved back by cutting out two pieces of aluminum and drilling holes around 10mm apart. Insert a 1/4" rod into the lens body, then place the aluminum pieces over the rod, and insert the scissor strut through the other holes. This secures the left side of the lens body back around 10mm and to the right around 3mm. Another piece of aluminum is screwed to the right side of the lens body under the strut assembly. Try to keep the lens body parallel to the film plane.
Moving the lens body back keeps the rangefinder working correctly. It also prevents the camera from folding up all the way. I trimmed off the magnetic latch and the top to allow the cover to fit over the lens body, though the viewfinder doesn't fold down and the cover only fits up to the viewfinder.
I had to make a minor adjustment with the rangefinder to set infinity. This can be determined with a "ground glass" (I just used sanded plastic acetate mounted in an old Instax film pack), comparing the image on the film plane with the rangefinder.