The shaft needs to be able to slide, and yes heat is no good due to the plastic stuff inside of it. I'm not as technical as Anubis on the product description of the plastic, but......the toxic fumes are indeed toxic and the plastic is supposed to be there. The shaft should slide freely enough to collapse it a couple of inches to get it in place.
I would strongly recommend against dropping the steer gear to get this installed. By the time your done you'll need a new gear AND steering shaft. Not trying to be a wise *ss about this at all, but stop before you damage something else. Do you want a stock shaft with a freshly replaced rag joint? I have several used ones that I've yanked for Astro shafts. Not for steering feel, but rather all of my turbo headers run next the rag joint that gets melty when the EGT's are enough to have the manifolds turn orange.