Played with the 30 Gibbs for a few years and really like this round. You will need to fireform brass properly to work in that chamber. I use the cream of wheat method.
As was mentioned before, you need to expand the -06 brass to 338 cal. Then run the brass through a '-06 die to reduce the neck back down. You simply keep running the brass into the die deeper until it chambers with firm bolt pressure. I like the bolt to meet resistance about 1/3 before full close. That way I know the brass is solidly against the bolt face and chamber.
I use 15 to 17gr of HP38/Win 231, any LR primer, and fill to base of the neck with cream of wheat. Stuff a piece of paper towel in the neck to keep this mess in place and fire the brass. What pops out is a 90% formed case. The shoulder will be moved but not sharp yet.
Prep this case as normal. I use a 308 Lee Collet neck sizer and spacer to size the neck. Any 30cal neck sizer will work. I don't like FL sizing if I don't have to.
I found H4350 Extreme and 165gr SST to be a wonderful combination. I also used CCI BR2 primers. My loads were getting 3000fps out of a 26" barrel with very good accuracy. You can use starting data for the '06 and just work up till you get the accuracy and vel you want. I was able to go faster but accuracy fell off. Was still a pretty toasty load.
Below are pics of the brass I used. If you have any questions, feel free to ask.