My 30-40 Krag has a big chamber and Remington brass is on the small side. I have the modern chamber and I think its roomier with less freebore than the original.

Does anybody know wich other cases that would be closest/best to resize from to get slightly bigger base diameter? The rim need to work with the Highwall extractor.

I have ordered one each of 7x57R and 7x65R to try for size.

Have you looked at .303 British brass as a possible source? I am not sure if it is long enough, or enough bigger in diameter to help, but it has a listed base diameter of .460", while the Krag is listed at .457" in "Cartridges of the World".

If you have a big pocket book .405 brass is about what you need if your chamber can handle brass that is .458 on the head. The rim is also slightly larger in diameter and thicker.
The differences may prevent .405 brass from working in your rifle unless you have access to a lathe. I have made a few cases for my .303 using .405 cases. My .303 chamber measures .460 at the head end. The sloppy Lee-Enfield head space also permits function with the larger thicker rim.
The .405 brass is also thicker than the .303 brass and is much heavier. A .303 case usually weighs about 160 grs. One made from the .405 weighs 195 grains. You may also have to turn the case neck.

For the larger chamber that you have you need to find a stash of twenty year old Winchester or Remington .30-40 Krag cases. They measured .455 on the head. Current brass measures .451 to .452. So do the current and also shorter .303 cases.