I don't think what you're doing is really feasible. The .308 has a larger case head by at least .013" which would cause problems with the chambering and extraction of the case.
Brass is available if you look.
https://nereloading.com/once%20fired...0shipping.html
https://gabrass.com/products/35-remington-fired-brass
https://www.armorally.com/shop/35-remington/