I will agree with all of those that mention the 6mmbr fast twist for varmints beyond 350yds.In fact the 6BR is one of my all time favorites.The 6br will easily hit 3800fps. in a 26" barrel with 55gr. Nosler BT's.....varmint vaporizer. and the 1 in 8 twist will hit 2900 - 3000 fps with 105...excellent for longer range.
Good brass is easy to get and there is a huge selection of 6 mm bullets,very efficient case and easy to load for.
other considerations, if you want something with a little more snort and a little different then you might think about these other options...
I have recently been shooting a .243wssm rouge .272 tight neck and all I can say is it is amazing.
115 D-TAC at 3175 makes for an awesome long range varmint rifle. My rifle is holding .5MOA at 1000 yds. and I actually shot a .3MOA group at 1000 yds. the other day. I have also shot 4 consecutive groups at 600 yds. holding .3MOA. As far a white tails the 115 D-TAC still has 1550 ft./lbs. of energy at 500 yds. and all this with just 47 grs. of powder. Here is a link to an article I wrote about shooting this cartridge to a mile. 1 mile 6mm
The 105 A-maxe's in this cartridge will also be varmint vaporizers.
As far as the 6.5's take a look at the .260 and the .260 improved. The .260 is a real performer on white tails and with the right bullet is awesome for varmints.
FWIW and hope it all goes well with your rifle build.