Cam advice for more torque from 340

When you say 10.5:1 compression ratio, is that the factory rating for your year of engine? I seriously doubt it is actually 10:5:1: actual static CR would be in the low 9 range at best with original parts, and you would need domed pistons to actually get 10:5:1 with stock chambered heads.

Run a cranking compression test first and report what psi you get; that will tell part of the story. Do you have any piston and head gasket info to share?

Have you gone through and optimized the ignition timing?

The XE2868 tends to be a good middle ground cam between top end breathing without taking away too much low end torque. The 'XE' series features steeper than average lobe ramps to get better lift versus duration, which is all good for improved low end torque. You an do better for low end torque, but more info is needed on what you really have for static CR. If you really have 10.5:1 static CR, then going to lower duration takes a lot of care. (But I will guess your engine really doesn't have 10.5:1...)

Also, Rhodes lifters will give you lower effective duration at lower RPM's so that is a 'quickie' option to help, assuming you have a hydraulic lifter cam. It supposedly has an effect like shortening the duration by about 10 degrees at low RPM's, so it won't do as much to help low RPM torque as changing to the XE268 cam.