Virtually any solder will work. Solders with silver in them are stronger, but I use regular rosin or acid core solder, and in 40+ years of flying I've never had a solder joint fail (even on giant scale).

The trick is to make sure the rod is CLEAN (preferably with sandpaper), and properly heated. As far as the heat goes, if the solder flows (as opposed to beading up on the surface), you got enough heat. Just heat the clevis, and apply the solder to the rod. Also, I recommend using a good soldering gun. Irons work for small electrical connections, but they take forever to heat up a good size chunk of metal like a clevis.