For my pre- and post- Heresy White Scars army, I've been refurbishing some old Space Marine Rhinos into other tanks.
I had a tan tank with no doors and a grey tank with the proper front hull so I carefully pried the tanks apart with an X-Acto chisel blade; and with a little sanding, I was able to re-purpose the treads and hulls into the configuration I needed.
Next, I added some plumbers putty into the tread doors, let it dry, then drilled out a hole for the magnet to rest. I glued on the metal bits to the hull and lined up the magnets for the sponsons. Once the glue is dry, I'll prime most of the tank white and the bolters black and start painting.
Behind it, you can see a crusty Rhino. That was the tank that originally had the metal Predator bits attached. Getting the metal off that chassis was a chore. The previous owner had used a lot of plumbers putty and a generous amount of glue and I don't think I'll be able to do anything this hull except use it as a burned out wreck. I might be able to get two wrecked markers out of it if I'm crafty.