I replaced my heater core before putting the car up for the winter. It wasn't all that bad. I don't have A/C and that makes all the difference in the world. The big key to taking the box out is to not take the fender off to get to the lower right mounting stud. I drilled this out with a 1/4" drill bit. That leaves all the other fasteners accessible. I then replaced that stud with a self tapping screw for the time being until I disassemble for paint in the future (although the screw could be left in permanent). The core was out in 10 minutes. What took me a long time was removing the doors on the box to replace the seals. That required some heat and some gentle persuasion.