I was considering getting new gear boxes for my Porsche KT. Are all the gearboxes interchangeable between the tanks, or do you have to use Pershing boxes for Pershings, and so on?

If you buy new type boxes for KT, they 'drop in'. You can use Tiger 1 boxes ,just swap over the mounts,as the final drive is the same length as the KT. If you use Sherman or Pershing boxes the final drive will be wrong length I think.

you can use all gearboxes from tamiya but Pershing ones. good luck, tom

Yup pershing gearbox are different, so you can swap the KT, sherman's and tiger I's gearbox among each other but notthe pershing.