A while back, Roger Haney put a 8-twist barrel on his PPC, and began working up loads for 105 and 115 grain bullets. I searched the archives, but couldn't find his posts. I *think* he was getting 2,850 with the 105s, and good accuracy with an old stash of Winchester 748.
Now far be it from me to say ol' Rog ever tipped the can too much... He got other powders to work; my tenuous memory is he got velocities in the high 2,700 region, and very good accuracy. He never did get the 115 Bergers to work as well as the 105s.
Seriously, since you need an 8-twist barrel anyway, you could chamber it up with a regular PPC reamer & throat it out for 95-105 bullets for 600 yards.
No new dies or chambering reamer needed. (Really, you need a 6mm throating reamer anyway. I like the spiral fluted ones Manson sells, that can be use either in a lathe, or with a T-handle.)
Just before Hawks Ridge shut down, Bill Shehane & I both decided we would make up an 8-twist 6PPC for the next season, & show the brethren what it would do. But then the range closed & we never did it. Now, that was for 1K. I think you'd be pleasantly surprised at 600 yards.
And if that doesn't work, you can always buy the dies & reamer for 6mm Major or an improved Major & run it in the existing barrel.
There are other cases with a nominal PPC case head size. I made a 6mm using the 6.5x54 M-S case. Capacity wound up at 52 grains water -- a little bigger than the 6mm/250. Either the 6.5x54 M-S or the Carcano is about like a 2" PPC.
Case head with the Norma brand 6.5x54 Mannlicher-Shoenauer is .448, and fits my Panda 6-PPC bolt just fine. Best to check though before laying out money. Though if you did have to turn the rims, it would be just a touch, & no need to deepen the extractor groove. Wouldn't work in a drop-port, obviously.
At the time (& could be now) Norma cases were quite cheap, and very good. There is no need for very high pressure.
3,200 fps was there, but best accuracy was with H4350 right at 3,050. And by accuracy, I'm talking .200 groups at 100 yards with several brands of good 105 grain bullets.