this is just my opinion, but I think if you go with a broughton 5C barrel 26-28" long, throat it pretty long and use lapua brass, I'm pretty sure you could push a 140 to 3100+ in a 6.5/06 AI.
If I had it to do over again I would go that route, the kreiger 8 twist on my gibbs is a fine barrel shoots anything very well but it is a lil slow compared to a broughton or hart due to being a cut rifled barrel. and the 6.5/06 AI brass are easier to form, just neck them to 6.5 or else use 25/06 brass, load'em and shoot'em. about any 6.5 that matches 270 velocities other than the 264 wm is a wildcat.
since there is no way you can match the velocity of a 270 with an 06 based case with the same weight bullet of smaller diameter at the same pressure, you either must shoot more pressure or use a longer barrel. to give you an idea of how the gibbs outperforms the others with the same case here is a gibbs round compared to a 270 case
Its basicly a 270 necked to 6.5, shoulder moved foreward to give more capacity and changed to 40 degrees