Very very few 45cal bullets will exceed .300 BC. To get there you will need to shoot a VERY heavy bullet or at least a 300gr with a very aggressive ogive profile. These kinds of bullets often don't play very well with sabots.

It much easier to achieve this goal shooting a 45cal sabotless with high BC 45cal bullets. Now if we had a better 45x40 sabot, it would be easier. A 40cal 240-260gr will easily get you shooting long range without the additional recoil of a 300gr+ bullet.

Pretty much every company that makes a 45/70 barrel offers them between 1-18 and 1-22 twist which is just fine for long 40cal projectiles.