45LC would make a good option for subsonic loads, though it’s not hard to develop subsonic loads for a 45/70. The main advantage the 45/70 has is a wider range of bullet choices, eg like Hornady Sub-X bullets.
Most of the heavyweight 452 jacketed pills were probably designed for the 454 Casull and are unlikely to expand at subsonic velocity.
This is easy to address if you cast your own pills. A few suppliers make 452 moulds in 350gr+ weights with HP options. Just note the nose to crimp groove dimension as a number of 452 moulds are catering for revolver users who have long cylinders (eg Ruger). A 452 350gr WFN bullet with a HP, cast soft would make an awesome subsonic pill for a 45LC.
