The last time I tried modding a Strikedagger was long before I bought a 105 Dagger, so most of this is totally hindsight---I literally broke the Strikedager kit trying to put some sort of articulation into it--it's in multiple pieces in my bits box.
But essentially, the 105 Dagger, like all Seed-era kits, uses PC-123 "D" polycap to join the waist and the upper torso. When I said there are numerous ways to do this, I wasn't kidding:
1.) The simplest and messiest would be (after cutting off the lower torso of the Strikedagger)
cutting up the inside of the Strikedagger torso just enough to make the "D" polycap fit, superglue the surroundings, and let dry.
2.) Another (more refined)
way would need polystyrene sheet or strips, a drill, and some needle files. You make holes in styrene strip so that the polycap's 'joint buds' can fit in, basically cover the polycap in a polystyrene 'box' casing. After doing that, make a same size/shape square hole in the Strikedagger body, then use polystyrene cement to attach the part. Since styrene actually melts plastic, this is slightly more scure (and neater) than the first superglue-fill method.
3.) Third way would require some modding: essentially build the lower half of the upper torso of the 105 Dagger, and somehow modify the Strikedagger upper torso to fit. Depending on your skill, it could be a clean cut-and-glue, or else use lots of superglue and putty.
There's a couple more ways I can think of, but they escape me at the moment, and I wouldn't recommend them unless you've got a lot of experience in modding kits--and by that I mean Hobby Japan level skills.