I’m not sure which torso to transform to legendary for my heat build
Both are good choices, along with Zarkares.
You can use this: http://community.tacticsoft.net/t/the-new-post-nerf-maxed-mythicals-listing/6499 to see which torso would fit your build better.
I personally would recommend a Zark torso if you have no myth HP plates.
Windigo, is better