For the ' 68 ( Painted indexes ), I'd choose a 22 mm Tan Strap, in Calf, to match with the indexes, or a Cognac one...
Ala Paneristi...LOL
For The '65, the best of the best would be a kind of Pastore black calf strap, again ala Paneristi...
As the buckle can be taken off the strap, it won't be difficult to find this kind of strap, IMO....
Best!
Nicolas