Most military surplus Mausers I can modify from 8x57 to 30-06 length [3.34" cartridge length].
But the Oberndorf Turkish Mausers made between 1903 and 1905 in Germany have already been lengthened from 7.65x53 to 8x57 by the Turks in the 1930s. The feed ramp is cut further forward. The magazine is cut, lengthened, and brazed. There is a semicircular notch in the large ring so stripper clips of longer ammo can be used.
The K.Kale Turkish Mausers can be lengthened from 8x57 to 30-06.
Both were sold by BIG5 for $50 as Turkish Mausers 18 years ago. They were still in the Century cardboard boxes.