I suggest to not waste your time with the old ones. Cut them and slide the clamps out, this will ensure you dont crush the heater core tubes in the disassembly. Silicone lines dont grab onto the metal as rubber ones do so this surfaces needss to be as intact as possible.
Maybe take a wire brush or some scotchbrite to the aluminum heater core pipes to clean/ roughen up the surface prior to installation.

P.S. getting the heater cores off the engine side will be even more challenging that the side by the firewall.

The line going down behind motor on the passenger side was wet when I tried remove it yesterday. Didn't think much of it considering it was raining but that's probably it. Once I get the scrap blazer out of my garage I'll start on that. Hopefully this week. Brass does seem like a good choice over plastic, might have a fitting at work I can use. I've seen some people use those cable clamp plier things to get at the stuff up against the firewall. Might get some of those too.