I’m having trouble picturing what your plan is, and don’t see any ascii art. The name for the brackets you are looking for are “joist hangers”. Yes that is within the load limit of most joist hangers (2x6 is 400 lbs min usually), as well as a 2x6 8 ft span on edge (424 lbs), but double check your specific materials. Also FYI joist hangers need to be nailed into both the joist and the header.
I'm not a structural engineer or framer, but my intuition is that the weakness here would be the bag swinging and loading the 2x4 sideways, perpendicular to its length, and also torsion. It might help to use a 2x6 with a 2x4 bolted down the center in a T shape with the bag bolted through both. Maybe you could rest the 2x6 on top of a top plate with the 2x4 hanging down and bolt it through, maybe with a joist hanger too. I'm not sure how happy a joist hanger alone would be with the lateral and torsional load of that bag swinging. I don't think they're designed for that.
I’m having trouble picturing what your plan is, and don’t see any ascii art. The name for the brackets you are looking for are “joist hangers”. Yes that is within the load limit of most joist hangers (2x6 is 400 lbs min usually), as well as a 2x6 8 ft span on edge (424 lbs), but double check your specific materials. Also FYI joist hangers need to be nailed into both the joist and the header.
OP I'm afraid I can only offer thoughts: please try this and report back
I'm not a structural engineer or framer, but my intuition is that the weakness here would be the bag swinging and loading the 2x4 sideways, perpendicular to its length, and also torsion. It might help to use a 2x6 with a 2x4 bolted down the center in a T shape with the bag bolted through both. Maybe you could rest the 2x6 on top of a top plate with the 2x4 hanging down and bolt it through, maybe with a joist hanger too. I'm not sure how happy a joist hanger alone would be with the lateral and torsional load of that bag swinging. I don't think they're designed for that.
If it's going in the basement, why not just hang it from the ceiling? Are the joists exposed?