I have a sako 75 in 7mm ultra that I'm considering rebarreling to something bigger. Does anyone have information on the 375 ultra necked up to 458?

7mm to .458", that's definitely something bigger!
No real info, but check out the Vincent Long in the reloading section of AR. Basically 500g/2400 safely. There's also the 458 (450?) G&A mag from some years ago, 80's maybe.

Try this

http://www.z-hat.com/Ultracat.htm