You do not want to have normal thermoplastic insulated wire right up against the resistor or the tip of the J-Head. The stuff is likely rated at 85C or so. Your tip may be running as high as 230C. The wire leads on the resistor do not cary heat all that well. They are long and skinny. The temperature at the end of those leads is well below the tip temperature. The shrink tubing that people normally use is quite happy at the temperature the leads get to. It's only as you exit the shrink tube that the temperature spec on the wire applies. The idea is that you solder the "black wire" to the very end of the resistor leads and put the shrink tube over the solder joint and over the "black wire". The resistor leads are insulated with / wrapped in the kapton tape.
If the resistor leads are short enough that the wire out of the end of the shrink tube will be up against the body of the J-head, that's not good. If you can tape the resistor leads to the body of the J-Head and have the exit from the shrink tube away from the body of the J-Head you are probably fine. If you are running a fan to cool the head (a good idea) it will cool the leads as well. In that case you are very unlikely to get things very hot.
