the 303 Velox

7,65x53 Mauser

http://municion.org/

if this is correct your main problem is base diameter and shoulder diameter.I know the thick base problem from my M 88 rifle in 9x57 which does not accept new brass. modern 8x57 IS brass is around 11,90 on the base so I lathe turn it down to 11,70 mm whats a simple job.
Norma 9,3x57 brass is similar small on the base but maybe not 11,65mm so there will be no basic brass you can use without reducing base diamter.
because its 56mm long you better start with 8x57 IS, 30 06 brass needs maybe neck reaming.
imho, the best is to order a die set for the 303 Velox from ch4d this will give you the right shoulder.