The 338 RUM and 338-378 WBY will give comparable ballistics when usedwithlongers barrels.You might give the Weatherby Accu-Mark a look oranother option would be getting a Remington Sendero in 300 RUM and having it re-barreled to the 338 Ultra or finding a Sako action and build a Lapua from the ground up.
I think 30 inch barrels are going to be need to extract all the velocity you can out of these big cases.

Either of these are going to be a cheaper alternative to the $3500 TRG. The accuracy of course is going to have to begreat when you are talking extended ranges....and that is going to run into a little money, no shortcuts there.