Alright, I thought it might have been a typo - (in which case, had he been referencing the 358 Norma, you can just use 338 Win data with like weights and, increase the powder charge to reach same speeds. That is all within the realm of safety, and could be improved upon. Obviously, a 35 caliber in the same case will safely exceed the speed possible with a like weight in a smaller bore.
For a different case altogether, there are several bigger .338 caliber cases. I'd choose the one closest or slightly less in volume, and work with that data.
