Recknagel and Ziegler have quite a range of claw mounts available off the shelf. Recknagel at least is available from NECG.

You don't necessarily need to fit the dovetail base into the receiver, rather get your gunsmith to machine either a clamp ring which goes around the start of the barrel before the knox form or you can machine one up that can be soldered on. The barrel is smaller than the receiver so there should be enough height difference to fit the dovetail base.

Remember too that normally when fitting claw mounts, the bases are fitted first and then the rings are machined and filed to suit the bases which are already fitted. They have to be set up with a small amount of pre-tension in the scope so that the rings snap in and don't wobble around. I feel that this would be quite difficult to achieve with a set of rings that have already been fitted, especially to a Mannlicher where the rear base is screwed in from the side and on an angle. Still, I am sure that Joel Dorleac will be able to guide you as he is the master of the claw mount.

If you do decide to still machine the receiver remember that you will loose some of the lovely markings on the receiver which is what makes a MS so special. I would make sure that the gunsmith gets the dovetail cutter ground to have a 0.020" radius at the corner which will leave your receiver significantly stronger, even small radii make a big difference in a piece of highly stressed metal to resist cracking. You will have to match the radius on the edge of the base but that is easy.

Waidmannsheil.