Originally posted by Levi the problem is that the distance from the rear lens flange to the film is shorter than any camera currently in production.
Levi
I imagine that you mean further out than any other camera.
At least from my understanding shorter would mean that you could add a shim to bring it to the right distance.
However you probably wouldn't have to, unless the lens sits too close to the mirror where it won't open or close properly.
You'd just loose your close up focus.
If the lens sits too far out then you'd never get infinity.
However if you can adapt a lens to fit your Pentax it might make one wicked Macro lens.
It'll allow you to focus closer than you possibly could on the old film Canon.
It's a little like adding extension tubes
I learned the above when I started to modify some of my Pentax glass to fit my Sigma. It's sensor is about 2mm closer than the Pentax
Edvard_Grieg,
If you can't work it all out look into getting a good quality 1.4x teleconverter that will fit the lens.
Preferably a brand like Sigma or Tameron, where they make more than one mount.
Plus buy the matching model, in a Pentax mount.
If you take these both into a machine shop they'll most likely be able to machine a hair off the bottom of the canon mount, and fit the Pentax fitting.
Or it might even work if you just fit the Canon fitting onto the Pentax TC, however I can't promise that this would work
Good luck, and have fun.