To get 155's out the barrel fast enough to remain supersonic through 1000 yards, you'll need a 30" one to shoot 'em at least 2950 fps. 175's will be just fine leaving at 2600 fps.
If your barrel's groove diameter's a few ten thousandths smaller than bullet diameter, your chances are excellent the 175's will shoot very accurate.
1) You will be able to get my 24 " barreled 308 to shoot 1k accurately and consistently. Try 43 to 44 grains of IMR4064.
2) That load will likely work for all ranges from 100 to 1000 yards.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master