I am in process of doing some rust repair on my 64 short bed door pillars, rocker panels, floors etc. While in process of completing these repairs I plan on spraying a product called rust destroyer into the areas that will be inaccessible after repairs, for example up into the door pillars. I also am trying to locate door pillar repair kits for the 64 but have only found them for 67 thru 72 and have read where these have been used for this repair with some cutting/modification the supplier says they will not work even with cutting I have not put my eyes on them yet to confirm. My questions are as follows.

1) Is anyone familiar with this product (rust destroyer) and does it work I have not seen it discussed on this website?
2) Does anyone know where I can purchase a door pillar repair patch for a 64 chev truck?
3) When replacing my front fender/door for example the area where the fender overlaps the fenderwell and the areas between the cab and door hinge is it acceptable the paint these areas with something like rustoleum to prevent them from rusting after the truck gets painted? I am trying to figure out the best way to handle these areas without having to disassemble truck at time of painting, I am not intending to disassemble before painting.